first of all,let me give you a hug……
With my high-functioning autistic adhd DD, I have run into similar problems as well.
So one thing I used to do was to invite our family friends and relatives we are close with,yet keep it interesting for my dd with her choice of cake and decor themes etc.
It worked for us a for many years until she grew out of it.
now for certain kids ,having that much crowd maybe overwhelming or it might turn out to be expensive for parents.
just come up with some unique way to celeberate birthdays esp with a focus on what your child likes to do.
she now has one friend ,who also has adhd, and they like to go to nail salon and get a pedi together and have some tea party in the backyard later…it just works for her.
Now ,with my dd, I would have to mentally prep her if something like this were to happen to her,otherwise it would be a huge tantrum throwing fest .
If your son likes going somewhere like fun rides at a park etc,that would be another option.I hope you find a solution that works out well for all of you.