On toilet training:
I personally think it's more confusing for the kids to go back and forth. If he is asking for underwear, put him in then do not make a big deal out of it if he has an accident. Keep him in underwear no matter what (since that is what he wants anyway). DD (25mo) still wears a diaper at night but often refuses at naps. Our deal is that if she goes before the nap she doesn't have to. For nights I still make her because she is in a crib and I would think it would be horrible to have to go and not have any access to the bathroom (assuming she even woke up when she had to go which is doubtful in itself).
Preschool:
Is there a local preschool you have a connection to? Do you have an older child goes, or a close friend's kid, siblings kid? If so, I would see if your son could go to a "trial day" to see if it is everything he thinks it will be.
