I'm so sorry you are going through this! This happened with my DS in 1st grade last year. He's not subject accelerated but they use AR at his school and reading levels are pretty public and he's the highest in his grade. So he felt like he had a target on his back at the beginning of the year last year. He caught a lot of flak.
He is in a large school we had a lot more than 5 boys to choose from, but this still might work. When he told me something negative, I'd say, but are ALL the boys saying this/acting mean/excluding you? And of course, he'd say no. I'd say, well who was nice today? Let's focus on the boys who are nice. And I'd invite one boy over to play. After 4 or 5 play dates with different boys either in his class or in other first grade classes(over a period of a couple months) he was feeling more like he belonged. The second half of the year was positive.
Obviously with only 5 boys in the class, it's a smaller group, but maybe you could still invite them over to play, one at a time, so that they can get to know your son away from the peer group.
You can work with the teacher and the administration, and definitely ask them for tips but when kids do or say something mean to another child, they don't always do it in front of a teacher, which makes it difficult to monitor and correct.
