Oof, reading your post broke my heart. I have ADHD myself, so I thought I would give you my perspective as someone who went through some similar things as a kid. I’m not a parent, so I don’t have any practical advice to offer, but maybe these insights will be helpful in some way.
Maybe encourage your son to think about how he defines “a friend,” and about the different roles people fill in his life. My mom always encouraged me to do that when I was down about friends. I would be heartbroken because Sarah or Emily didn’t want to be my best friend, and after I thought things were going so well! We always played together on the playground! My mom would tell me that, well, maybe that’s just the kind of relationship you’re gonna have with Sarah and Emily. They’ll be your playground friends. Mom would remind me that not all friendships have to be these deep, life changing relationships. We all WANT those kinds of friendships, but they’re hard to find, even for people without ADHD. I’m 24 now, and I have lots of people I can call to go out for a drink with. I have my work buddies. I have my old grad school buddies. But I only have two “real friends.” Two people I have really strong emotional connections with. As a kid, that would’ve really frustrated me, because I wanted that kind of relationship with everyone I met. Now that I’m older, I realize that most people only have those kinds of deep connections with a handful of people.
You mention your son picks up on rejection, and he sounds like a very empathetic kid. I am pretty similar. It’s really easy for me to pick up on peoples’ emotions. But it’s NOT easy for me to distinguish the cause. When I can tell someone is angry, uncomfortable, bored, annoyed, etc., I tend to always assume I’m the cause. And that translates into rejection for me. And sometimes I am the cause! But the truth is, someone could be feeling a negative emotion for any number of reasons. Is it possible your son is sometimes seeing rejection where it might not really exist? I notice this more and more in myself the older I get. There were times as a kid I would feel so disliked and unwanted by certain people. Then I would find out years later that they actually thought I was cool, and wanted to be friends, but I avoided them or was standoffish because I thought they hated me!
Good luck to you. I hope things get better soon for your son.