Come on, what are the wider circumstances?
Is this birthday child an anti-social child or being bullied?
Just how is your child being influenced and by who?
Is your child being bullied or just feeling insecure?
I'd be the protective - from a distance - parent.
I'd stay for the party and get to know the parents and other parents staying.
If there's a problem identified by DS for not wanting to go to party - I'd want to know why.
I'd exactly be there. Be friendly, look to make friends. And just keep an eye on everything.
You'll work it out. But it's probably the easiest way to understand the problem.
It's a huge advantage to son to have supportive, friendly, loving mum at the event for reassurance.
Just highlight all the fun stuff of going - and if he's miserable, you're right there. You can just say he's a little out of sorts that day, and probably best to take him home.