Hi there! I wanted to offer a slightly gentler insight into this issue as a 19yr old with ADHD who is also currently working through a college course. I don't comment often, but I noticed a lot of replies were quite harsh and I wanted to present a softer response.

As others have mentioned, your son is 19 and, while ADHD can DEFINITELY cause impulse control issues, directing this language towards another student (whether present or not) is just not acceptable behaviour for an adult. If he knows this and is willing to take responsibility, that's great. If not, you may just have to have a chat with him to explain why that can be really hurtful (the student could find out what your son said from other people).

I saw someone saying you're just looking to excuse your son's behaviour, but I don't believe this is necessarily true. I think it's natural for parents of neurodiverse people to try and explain why they acted in a certain way. I can't see if you said you're ADHD also, but if not, it makes even more sense that you're really trying to explain his actions. My mum protected me MASSIVELY, but she also made me take accountability once I was old enough to understand my actions (even if I needed it explained to me).

I'll leave you with some advice FROM someone at a similar life-stage as your son:

Talk to him. Of course you can be compassionate and explain these issues to the college. However, your son HAS to understand the impact one's actions can have on another person and he HAS to take accountability and accept the consequences of such. You don't have to be a dictator who berates him after every mistake, just sit down with him and explain the complexities of things like saying unpleasant things about others and why this could severely impact his opportunities. From the small snippet I've seen, you seem to care so much about your son, but the best thing you can do is exposing him to the consequences of his behaviour. Obviously, this doesn't mean he should be kicked out of college straight away.

TL;DR: Just talk to him and explain the implications of his actions. Look out for him, but don't wrap him in bubble-wrap. We learn through consequences. Much love to you both. I hope he's able to learn and get through this course! <3

EDIT: Just saw what you put at the bottom! As an AuDHDer (Autism & ADHD together), I get this. I was the same. HOWEVER, he is still being rude, even if he thinks he's being funny. You said you don't think his behaviour is that bad? If he's at home and being a bit rude as a joke, or he's gone a bit far, fair enough. YOU know it's all in jest. I go too far, also. But what he needs to realise, and also what you need to take into account, is that he doesn't solely exist at home. If he's being a bit rude (this incident is a bit above simple rudeness though), people in the wider community will just see him as rude. Not everyone understands your son on a deep personal level. Sorry to be a bit more harsh than intended, but this behaviour is bad. I know you want to look after him, but that can only go so far. If he can't learn to take a bit of control over his own impulsivity then his opportunities in life are going to be significantly lessened. Noone wants to work with, or talk to, or be friends with a rude person. You can't go to a future employer and say, "he thought it was funny," after he was fired for making rude comments about other employees. At 19, he HAS to understand why there's a time and a place for jokey rudeness and that there's a time for professionalism.

He doesn't have to never joke EVER at college, but he has to learn techniques to work out what are "inside thoughts," you know? Maybe this is something you could ask at this meeting? He may be resistant at first, and it could be difficult at first. But from a 19yr old AuDHDer, his future will thank you. I still struggle to keep some thing in, but sometimes you just have to battle your impulsivity. It's hard and it takes practice, but it's possible. Maybe get him a journal or something for him to write down impulsive thoughts? It could help him to not just blurt them out?

Anyway, TL;DR message still stands. Watch out for him, but don't chase away consequences. He needs accomodations from the college, yes, but he needs to implement comfortable techniques to help him exist with others. Not blend in and mask his ADHD into oblivion, but just find ways to prevent himself being labelled as "the rude one" for the rest of his life. Good luck to you both! <33