Aeh and
I'll offer my opinions since I went through something very similar when thinking about pre-k. We had a lot of the same concerns.
1) Definitely tell them how advanced he is. We started at the top (curriculum administrators) and gradually worked down to the teachers. Give them concrete examples. If you have things from home, bring them in to show them. I found that teachers and administrators responded better to showing them something my son could do compared to simply telling them.
That said, if they say they can differentiate for your son then definitely ask for examples of how they would do that. When we were looking for pre-k's that was always the make or break question. I felt that the schools who genuinely knew how to deal with a gifted kid of that age could give you off the cuff examples or refer you to someone who could. The schools that didn't know how, they frequently told us "Oh, don't worry we have lots of kids like that in our school..."
2) I agree with Aeh, if they can't really do exactly what he needs then half-day is better. When we first ran into this problem, I took my son out of the daycare and my wife and I taught him from home. We sent him back 3 days a week for 6 months before pre-k because we wanted him to get familiar with the group setting again. We both felt it ended up being a tremendous waste of time. There was some socializing skills which were valuable but he stagnated everywhere else (we just didn't want to turn his time home into catch up sessions).
I realize that it's anecdotal but I hope it adds some value.
