S.L.
If your child is learning new concepts, becoming more independent, learning to be polite, enjoys this daycare, and seems happy and well adjusted, then I see no value in moving him. It is quite possible to get a list of skills needed before he starts school and you could run down this list and make sure he's ready in about 10 minutes per day. Just a quick run down of the list daily would give you an idea of what you could work on for the day.
The most important things I think children need to learn before school is all about morals. It's hard to teach morality to such young children. But it should be modeled. It's not that a preschool is detrimental to this goal. But in some cases the large size of a preschool just naturally exposes a young child to so many more negative attitudes. The potential for turn over in the employees can also cause a disconnect between the caregivers and the child. I think in the first 5 years children need more of a bonding between them and their constant caregivers. They have the rest of their lives to learn to be outgoing and brave in the face of constant strangers and strange situations.
BUT, all that aside, some kids would truly benefit from being in a preschool. If your child tends toward bullying behavior, it would be good for them to be exposed to situations where they are not able to automatically get as much one on one attention. I am afraid that children in small intimate settings are just catered to more and it causes them to have a self grandeur or self importance feeling that can cause them to be rather selfish. But not all kids are like that.
Some children thrive on routine and structure while others thrive on a loosely planned day. In the end it is your judgment call to make as you know him best. :)
Suzi