One thing that really helped my sensitive 3 year old to adjust was a family "photo poster" that we hung in her cubby low down where she could go look at it whenever she needed to. It has pics of me and DH and her sibs and grandparents and our cats. Knowing she could go and look at it really helped, and it made her feel special since all the other kids wanted to come look to and wanted one for themselves. Plus, it helped her teachers to see that we really cared a lot about making her feel comfortable, and gave them something that they could talk to DD about when DD was upset. Getting her to tell stories about her family helped her to feel more settled in and understood.
I agree that maybe the star system might just encourage him to bury his feelings, while not really solving the root of the trouble. I would probably do a little investigating about what time of day it is that is the most trouble for him, and do some poking around to make sure there's nothing solvable that's bothering him, before resorting to something like that.
me , he , my three , , and -- and the one we lost