I can’t tell if this other kid is just an example of how all kids treat your son, or if it’s just that kid. If it’s just that kid, then you really should confront the parents directly and sever the relationship if they’re unwilling to work it out with you.
Keep in mind, though, that they may have an issue with you that you’re unaware of, and that issue is what may cause or at least contribute to the problems you see. We all think our own kids are better than other kids in some way, shape, or form and we’re highly prone to closing our eyes to behaviors that others see more clearly. So, be willing to accept feedback if you’re going to give it.
If this is a more general problem, then you probably can benefit from an evaluation of treatment. Perhaps a second opinion if his current doctor thinks everything is fine. For example, he may need some cognitive therapy to help him better learn social skills, or an adjustment of medication.