Feel for you. People sight small clasess as A Good Thing for kids, but so often, for this very reason, they aren't.
Ds was in a class with only 3 other boys! Luckily 2 of them were on his wavelength and all 3 became best friends.
However it meant that the 4th child was always left out. As the parents of the 'popular' boys, (remember DS only had 2 friends!) us other mums were really aware of this and made every effort to invite this 4th child to tea, always included him in birthday parties etc.
But the problem was, he just wasn't into the same things as our boys, and although they tried to include him, it just didn't work - from his point of view as well. He wasn't particularly interested in my son and his friends!.
I think what I'm trying to say is that you can't force a friendship, however much you try.
The 4 boys are all now at high school and the 'left out' one has found a good group of like-minded boys as there are so many more to chose from.
Don't let the fact that your Ds isn't sporty be a red herring. My DS is super-sporty and outgoing, one of his friends was very shy and one was very arty. The point is, they clicked on some level and so embraced each other's differences.
You're right, your DS needs to find just one like minded soul - all you need is one friend! It sounds like his old BF is more outgoing then him and is exploring his options.
Since you only have 11 other boys to chose from you could actually be quite systematic about seeking them out, inviting to tea etc. Perhaps there are some obvious ones who you know wouldn't get along with your DS, but there may be one dark horse!
Eye up the mums - see if there are any like-minded souls or anyone you might chose as a friend - chances are if the mum is on the same wavelength, the kids might be too?
And don't discount the girls. They may seem tight knit but Ds 1's best friend in reception was a little girl. Perhaps invite one of them to tea?
I think you are right to be pro-active about it - and right to help your DS out.