That could be a lot of things. Maybe you could keep a journal of his behavior? And then present it to a medical professional. Plus, he needs someone to advocate for him. I say this because you want to be certain that he gets the right care.
Pay attention and speak up if something isn't right. You've made a good first step by coming here.

weird question:
Why do you specify that he is not mean to animals? I am glad he isn't, but I'm not sure what it has to do with what you are describing.