My son was not invited to the party ~ How best to deal with this?

I agree with Stanley. You never know what the cirstances are. Maybe his mom said he could only invite so many kids? I wouldn't blame the parents either because it could have been that their son just didn't want him to go for whatever reason. I would just go about the basketball game/practice as usual and tell your son they are missing out and try to make it not a big deal.