I would tell you to meet with his teacher and ask how he's interacting throughout the day.
Just like adults, lots of kids just "aren't morning people." Parents are so busy nowadays, and mornings can be a stressful blur for children, not to mention that many parents don't enact a bedtime either. So while your child is well-rested, calm, and happy, the other children could still be tired and cranky.
It just doesn't follow for me that he would want to go to school if he doesn't have friends. Kindergarten was only 15 years ago for me, and I would certainly have fought tooth and nail if I had been lonely there. If your son makes friends other places, he
should be able to make friends there. I hope it's just a situational problem for him.
Good luck