My son is autistic and I felt like small classes were a key as well, but also I wanted the consistency of the coach.
We registered for a class at Norwood park with Kids Pro soccer nearly two years ago when my son turned 3. He has been taking that class for two years now and we have the same coach-coach Jo who is amazing with my son.
He knows him and memorized the whole routine that gives him so much confidence that nobody can notice his social inhibitions anymore. Now, he is five and I frankly don't know what I would do without this program that seems to be the only group activity that is not stressful for him.
I frantically check before each session who the coach is!
I definitely think that your child could benefit from that class. From what I have seen on the website, they also do classes per request, like with only his friends etc. so you can limit the number of friends you are inviting.