Hello any advice will be appreciated.

I have a 4 year old boy who will be turning 5 in June and his ‘best friend’ at school is really mean and bossy to him. His friend is 10 months older than my son and he tells my son what to do and reminds him everyday that he is number one at school, according to my son. Our families are pretty close, and I’ve seen them quarreling couple times but didn’t think much of it until recently when I saw their interaction up close. My son and he were playing in the playground and when my son was about to go down the slide his friend stopped him saying he needed to go down first. When my son was on this alligator ride his friend told him to move to the back because he always gets the front seat. And my son actually moved back and this kinda killed me.. 😢 It sounds pretty petty but my son almost seems defeated and he tells me his friend makes him feel bad very often. These last few days he came home crying. But school doesn’t think much of it because they’re “buddies”. My son’s showing some peculiar behaviors too, like he would try to put me(his mom) down like “I’m better than you” or “i can do xx but you can’t”

My son is having a birthday party in June and I’m afraid that his friend will make him cry again on his special day. If i call up his mom and ask her to remind her son to be nice to my boy, I know it will make her uncomfortable and feel like they’re unwelcomed at the birthday party. She never reprimands him so will definitely get offended if I ask her of this. I don’t know what to do? I keep telling my son to walk away when he makes him feel bad but he sticks around because he loves his friend. What can I do so that my son will have a happy birthday?