I hope the party worked out…. you perhaps still could..or next year could have a party after a baseball game that includes his teammates. Cake, pizza at the ballpark, or hotdogs from the concession stand and bring a cake. You could do invites to all the team saying “Celebrate ____’s birthday after we play the Cardinals!” That way everyone is already there, knows about it, and doesn’t even have to rsvp. It’s a ready-made party. Happy Birthday to your son!