Aww... poor little guy! Please give him an extra big hug for me!
I feel for his parents, too. How hard it must be for them, having the challenges of this new baby and dealing with this problem on top of it!
And I know it's hard on you GPs, also. It's great that you're there to give him one-on-one attention - bless you all! But that means you're very close to the situation, so I know it's affecting you.
Glad that GS opened up a little to his dad and that things seemed to go better as a result. I believe GS about missing his old teacher. Each of my DDs had a favorite teacher leave in the middle of the year, at one point, and it impacted them very strongly for a while, one DD in particular.
However, I'm sure the fact that there's a new baby home with mum has something to do with it, too. He may seem to have "accepted" him. but he also seemed to "settle in well" to the new school. And he did, judging by what the teachers say, but still there was something bothering him, after all. So it wouldn't surprise me if a part of him was upset about the baby getting more attention from his mother (or so he thinks).
The fact that he's ok going to the gym class doesn't cancel that out, IMO. He probably enjoys the gym class so much that it overcomes any misgivings he has about the baby being with his mother. And he may be aware that the time at gym is much shorter.
Hopefully, he will keep talking and letting his parents know what's on his mind. (Is there any chance for family counseling?) Mostly, I just hope this keeps getting better. Actually, I'm sure it will.