I'm so sorry.
I know it's late in the school year, but call the school's guidance counselor and insist that he/she help. My son got to have a few weeks of a weekly lunch buddies with 2 or 3 of his besties in our counselor's office. They were such a great group that she said she really looked forward to their time and would quickly break away from other meetings or conversations to protect that time. I know it's late, but your son needs it so much.
Also, can your dh go to school for lunch with ds once/week? It makes a huge difference. All the boys gravitate to the dads when they do that. It really raises the boys' social capital. The study I saw used college students instead of dads, but dh says that it works with dads, too.
Is your ds getting any kind of special ed services? Be sure to let those teachers know, too. In our school district, kids who are determined to have special needs might get social time, similar to the lunch buddies. Your ds has ADHD, doesn't he? That has very notable social implications, so he deserves and needs social time.
Insist on an observer to watch ds and this friend asap. You need information about how he is interacting with other kids at school. If he is shy, or if he's antagonistic or immature, this is all valuable info.
Get the dr to give you a dx. Depression, anxiety, etc. are probably grounds for immediate action. A neighbor's dd was dx'd with clinical depression in 5k b/c of a chaotic 5k teacher, and she got xferred to a better school right away.
What would you think about taking ds to a play therapist? We do that every other week. I observe during the sessions, and later, the therapist calls me and we talk about things I can't discuss in front of ds. DS took to the therapist right away.
I'm really sorry about your poor ds. I hope it will be sorted out soon. My ds's year is definitely wrapping up better than it began, and I sure hope I don't jinx him by saying that.
Keiki's Makuahine (Keiki's Mom) 51, dh 52
Keiki: b. 2002 after 3 months bedrest
Natural conception following ZIFT/chem. pg
Olivia: b. 1999 d. 1999
28-week preemie, ptl cause unknown
Natural conception after 1 mc
ttc since 1998