Oh man… I know how your little guy feels. I think I was only invited to maybe two birthday parties as a young kid, and one of them was a whole class kind of thing. I knew, too, that most (if not all) Valentines that I received were really from their parents. Like your son, I was awkward and not very well accepted. I moved a lot as well, which made things even more difficult.
I remember being pretty upset that other kids didn’t invite me to things… hell… not even play dates. So my mom did her best to make my birthdays special in other ways. They became family-only and I got to choose what kind of cake to have and what to have for dinner, AND rent a new movie. It was pretty darn fantastic.
If you can’t get his baseball team to come, or have the party at one of their practices, you could just say, “Hey buddy! We decided to do something even more special! We are going to go to such-and-such place as a special birthday treat!” I totally ate that up as a kid and my siblings loved it too because they basically got a treat as well.
As you can see, there are still some happy options. I probably wouldn’t tell her son until he’s old enough to not be crushed about why your original plans went through. I remember being pretty heartbroken in third or fourth grade because I couldn’t understand why most of my class wouldn’t speak to me. I was “odd”, and there simply wasn’t much to be done about my classmates until they grew up enough themselves to realize that “odd” could actually be pretty fun. Once I got into high school with a big enough pool of students (I lived in rural Vermont… my Union high school still barely had 100 in my grade despite having several towns to feed it), I found other “oddballs” just as harmlessly goofy as me and it was best buddies at first sight. I relate this more for your benefit than for your son’s, because I remember how sad and powerless my mother looked when I cried and asked why I didn’t have any friends. Even though my mom couldn’t “fix” what was wrong at school, she helped me find joy elsewhere and encouraged my interest in music, reading, and art. (I was hopeless in sports… the ground and me face loved to meet… often…)
Growing up with ADHD is pretty rough, but your son has one advantage that I didn’t… you both know WHY he is the way that he is. I wasn’t diagnosed until my mid-30’s, and so I quite literally always thought I was just “other” and strange until my diagnosis finally made aspects of my life make sense for the first time. Keep at those mindfulness strategies… he’s at the perfect age for them!
Good luck and I wish your wonderful son a very very happy birthday!