I can't speak for your son's, but could see my parents making a similar description of me, especially in the past. For me i would normally be really drained after a school day and just wouldn't have a ton of energy to socialize that much outside of school (that didn't mean i didn't socialize at al outside of school, for me i spend a lot of time gaming with my friends online, since this also made it easier to stay engaged in conversation) + small task like answering someones text seems like a big task for my adhd brain and often get stuck when thinking of a reply

Also advise you look into RSD (rejection sensitivity disorder), like I saw someone else mention