The short answer: it’s because he’s black and the people doing it are largely white and there’s cultural baggage surrounding white people using the words “boy” and “son” to address black men. 

The long answer starts out with the idea of tonedeafness and a fandom phenomenon that crops up when predominately white fanbases are exposed to dimensional, compelling characters of color. The same thing happened with Star Wars and Pacific Rim and so many other diverse franchises lately. 

A lot of the time, white fans are genuinely not trying to be racist, but most of their faves up to this point have been white, and they haven’t considered that perhaps the way they write and talk about those faves would take on different implications when the characters’ race is considered. 

For instance, and I get in trouble a lot for bringing this up, but a few months ago there was a Disney AU fanart of Finn and Rey from Star Wars as Tarzan and Jane. Now, in the movie, Tarzan and Jane are both white, but in the art, the impact changes because Finn is a black man and the artist drew him as an animalistic ape-man who meets a delicate high-class British woman who “civilizes” him. Obviously the Tarzan/Jane dynamic has a very VERY different meaning if Tarzan is depicted as black and Jane is depicted as white, and it is in fact racist to depict Finn that way even if it wouldn’t even be the smallest problem to draw, say, Iron Man and Pepper Potts in the same exact situation. (Also if anyone is Tarzan in that pairing, it’s Rey, but I digress)

So you get these situations where people are trying to do the stuff they always do for all characters, only their faves have mostly been white up to this point so they’ve never really had to consider the racial implications of the stuff they say and write about those characters. That’s why they draw D.Va as an infant without realizing that the infantilization of East Asian women is actually a harmful racist practice, and then when informed of this fact, instead of saying “oh shit, I didn’t know I was contributing to that! Thanks for telling me, I’ll stop doing it,” they get defensive and claim that actually it doesn’t matter if the end product is 100% identical to racism, because they didn’t intend for it to be racist, that’s not what they were trying to do.

Also, generally speaking, they don’t do the same thing to white characters. While jokes at the expense of Soldier: 76 and Zarya are usually things like “he’s old and grumpy” or “she’s really strong,” jokes about Reaper are more like “he’s got a huge dick and he’s abusive and a rapist” and jokes about D.Va are usually “she’s a dirty and mischievous subhuman creature and the white guy is like her dad.” The fact that a lot of people make all these jokes and think they’re roughly equivalent speaks to how much unconscious racism they’ve got to purge from their system. 

Alright, so now that we understand that, let’s get into a little more of why “boy” and “son” in particular are not the sort of thing you should not call Lucio. 

The first and main reason is that he’s a grown man, aged 26, but more importantly, he is a black man. Historically, the words “boy” and “son” have been used on black men for two reasons: 

1. Because even grown black men were to be treated as childlike under white supremacy, esp. under slavery, and even after the abolition of slavery, the words “boy” and “son” are still used in order to talk down to black men. You will still frequently catch younger white people address black men older than them as “boy” or “son,” especially in a service capacity (i.e. a black waiter or employee at a store). Under slavery, the dominant white supremacist narrative was that even the smartest black people were only on the level of white children, which is obviously a complete falsehood fabricated to justify their continued subjugation by saying “they’d be lost without us.” So, by referring to black men as “boy” or “son,” that’s the message that was being communicated, that even though any given black person is grown, they’re still viewed as roughly mentally equivalent to children. 
   
2. A lot of slaveowners didn’t feel it was worth it to learn the individual names of their slaves, so they would simply address them as “boy” or “son” (or “girl” or a variety of other degrading names for women) and this practice continued even after the abolition of slavery. Again, calling back to the “black waiter” situation I referred to earlier, you still sometimes see white patrons referring to black employees as “boy” or “son” in this way. For older people, they would use the terms “Auntie” and “Uncle” as a way to deny them honorific titles such as “Mister” and “Miss,” which is where we get mascots like “Aunt Jemima” and “Uncle Ben,” both of whom were derived from this practice. A similar example is how a lot of white railroad passengers wouldn’t bother to learn the names of their car’s porter and would simply call them all “George,” which again sort of demonstrates my point: the name “George” isn’t inherently racist, lots of people have that name, but to call a black guy doing their job that carries different implications even if you “didn’t mean it that way.”

So generally, there’s nothing wrong with the words “boy” or “son” most of the time, but when you address a black man this way, it carries a whole different implication. I’m not trying to condemn anyone morally or say “you’re evil if you’ve ever used these words about Lucio” or anything, but back to the beginning of this:

I am assuming you all have positive intent, that you are all well-meaning and that you are definitely not trying to be racist. Because of this, I feel like it’s my responsibility to tell you when a thing you’re saying carries meanings that you maybe didn’t consider and definitely didn’t mean to imply. I know I would feel foolish and guilty if I found out something I’d been saying casually actually had a racist meaning that I wasn’t aware of, so I just want to say that if anyone reading this is (like me) a white person who’s really truly well-intentioned and doesn’t mean to be racist at all, your response here should be “oh wow, I didn’t know that Boy and Son are names you generally shouldn’t call black people, I’ll be more conscious of that in the future,” and if your response is to become defensive and try to prove that it isn’t bad because you didn’t mean it “that way,” it either means you aren’t well-intentioned and do mean to be racist OR it means you didn’t read the post. 

That being said, I’m happy to inform where I can, but I’m also not black, and a lot of black writers have explained this a lot more eloquently than me. I suggest you do some googling and research what they’ve said on the subject, because I’m sure they’ll give you a clearer picture than I possibly can.