5 here

No some people aren't easily offended by rude comments, like I have friends who are totally ok with being rude to one another. Or if you say he knows what offends you then maybe he was just teasing you, you know he might believe in the saying that fighting with one another means you like them (which I don't really believe in personally).

No I don't think he will stop being nice to you if you be rude to him because he was rude to you first, and if he finds it surprising that you are suddenly so rude then you can just snap back and say it was because he was being mean first.

Some people are just very irritated and stressed out most of the time and so he may be that type of person or he's just not very agreeable. He also may be experiencing really stressful or sad events in life, so you could ask him about that.

Whatever the case, I think it's best to talk to him in person, that way you can see his facial expressions and emotions as he explains the situation to you. Yes you wouldn't be able to convey as much using just IM, you may want to use IM to schedule the meeting with him though. Say just invite him to have a drink with you or something and bring up the topic after you've talked about some general stuff.