A.P.
Volunteer! Next chance you get, make the time to volunteer in the class (special day or normal day). Get in there and join in. 1 of 2 things could come from this: you can see if "Bobby" is overpowering the room or if he is just misunderstood, not being taught some social skills at home, etc so you know if you need to address the teacher further on this. Also, it will help you find some good things to point out about Bobby to your son. If you can find the good things, you can teach your son to look for them too. For example, when the shirt statement you mentioned before arises - you can then respond, "Hey, maybe he'll wear that cool Bumblebee T-shirt! Do you remember it from the other day? It was awesome!"
Perhaps you can get lucky and "catch" Bobby's parents in the hall at drop off/pick up and comment something like, "My son talks about your son all the time!" and see if they also are feeling the same way/have the same concerns. If anything, it'll give you more info. about who this kid is, where he comes from, and how to proceed in the future.
You don't have to force them to be friends. Some people just don't mesh. But you should teach acceptance and understanding. Helping your son to find the good in the Bobby's of the world will make him a better person.
Best of luck!