Initially he seemed excited about the new friends he made and even invited some boys over after school. Apparently, the other kids teased him about his brother after they came over, and now we're not sure if we should push the issue or just let it go.
Kids can be cruel, and unfamiliarity is often at the root of the cruelty.
But this situation presents you with a good opportunity to teach your son some important skills, raise awareness among his classmates, and, in doing so, pave the way to a smoother future for both of your sons. Start by helping your son learn and practice what to say when people tease him about his brother. It also would be wise to engage your son's teacher in helping the class learn about individual differences and disabilities. In a small town children have fewer opportunities to experience a wide variety of people, so teaching may need to be even more intentional than in a more diverse setting. You also could reach out to the parents of the boys who came to your home, letting them know that you're trying to help the kids become more knowledgeable about and comfortable with your younger son's disability.