I'm confused, you say he likes being at school but also that he doesn't settle? Does he get upset as you're dropping him off but then settles once you're out of sight? Or is he just upset all day? What do his teachers say?
When my daughter started pre-school the first few weeks she loved going in and no tears at all, then when the novelty of it all wore off she started getting very upset as I was leaving, would scream the place down, it was awful and broke my heart every time.. But she was always fine after a few mins. Now she's 4.5 and at school fulltime, she still has mornings where she gets upset when it's time to say bye but as soon as I'm out of sight she goes off happily to play with her friends. In fact I see so many kids doing this in the morning, crying and getting upset but then being totally fine when parents are gone. When dropping off it's much easier to just do it quickly, the more you stick around when they cry the worse it is for everybody. I've always spoken with my daughter about how school is her time to be learning and having fun with her little friends and that mummy will always come and get her when it's home time. They just need a lot of reassurance.