Luckily our DS6 has a December birthday or we would've faced a similar crunch with fine motor and sitting still. In pre-school he was the kid wandering around, but it was due to curiousity and a lack of interest in stories well behind his interests. He still prefers the company of older children.
So, if my DS had had a different birthday I'd have wondered:
Will he get along better with the younger kids?
Will the younger class be focusing on specific skill building for fine motor control or is this just biding time?
When he reaches K, will there be acceleration throughout the year or is it just overall an advanced class? i.e. is there an increased chance he'll be bored in K.
