D.B.
These are the years when friendships tend to be in flux anyway. Elementary schools combine at middle schools and later high schools, kids are coming & going, and so on. I'm not sure this age is any trickier than other ages - each age has its challenges and stresses, but also its joys and opportunities. So I'm not sure I'd put undue pressure on him or presume this to be much harder than, say, the first year of all-day school or the later teen years when kids are contemplating heading in totally different directions from their classmates.
There's nothing you can do about the moving, so all you can change is his ability to handle it. This is the age when we build more resilience in our kids - we stay out of personal relationships, we hang back and let the kids deal with their teachers and coaches, and so on. They will be on their own in 5-6 short years and they must have the skills to cope with life's disappointments.
There's a term circulating among educators - it's a step beyond "helicopter parenting" and it's called "lawnmower parenting." Helicopter parents hover and swoop in when there's a problem. Lawnmower parents get out in front and mow down any obstacles so the child never even sees them. A helicopter parent grabs and comforts a child for every little bump and bruise, while a lawnmower parent walks in front to prevent any bumps and bruises to begin with. I'm not suggesting that you're doing this, but I am suggesting that he may be surrounded by kids whose parent are doing at least some version of it. As a result, he's not seeing peers cope with difficulties, and that may make him feel less capable of doing it himself. What he needs most is someone to convince him that he is able and strong, and that he will get through this. He made friends, he'll make more, and he needs to be open to different kids as he matures and his interests change/develop/evolve.
Sports are fine for some kids, but not all. Maybe he has other interests he's not even explored yet. Maybe drama or the debate club, maybe a community theater group or a social action club. If there isn't one, maybe he could start one. What a leadership experience! Other kids flock to leaders because they are fun and interesting to be around. I think, if you convey more confidence and less worry, he'll do better. It's great that he talks to you about it, so keep those conversations going. Just be alert to his normal and natural need to start to break away a bit and handle things on his own. Let him go when it's time to do so, as hard as it is to not be his mommy all the time preventing problems. It's a hard transition for many of us. But worrying too much can make it worse for the kid - they pick up on our emotions and it gives them something else to deal with. Please work to avoid that.