My son, 12, has aspergers which is kind of like the “opposite” of ADHD, but has the same effect socially (my husband is the one with ADD).
What we do each year is bring cupcakes and capri-sun to the classroom on his birthday so everyone can celebrate, then, plan a party and invite ALL the kids from the class (with RSVP). Often if you invite the whole class there is at least one or two other children who never get invited to anything and are happy to be included. (I once gave an extra cupcake to a child who was sitting in front of the counselors office. He said “no one has ever given me a birthday cupcake before” I almost cried).
If no-one can make it, or only one or two, (it has happened to us more than once) we opt for a movie and pizza or something else kind of special that we know our son would enjoy.
When it’s obvious no one is going to make it, just say, “wow, it’s a bummer, but everyone has really important stuff they already have planned for this weekend and cant make it to your party, what would you like to do instead?
You may end up at REI climbing a wall, or bungee jumping, or at lego-land, but it will be okay. Your son will have fun, have done something exciting, and he will know he is loved by his family.
That will be enough.