Hi K. -
I do agree, unless you have some concern that the preschool is particularly bad for him, then making it his choice is not the best option. 3 1/2 is too young to choose a school. It's old enough to choose which pair of socks or pants but not school...
As far as Montessori, though. Are you sure you can't afford it? My daughter goes to Discovery Montessori on Hawkins Blvd. Right now, they rent rooms from the Beth El Synagogue, it's right near the Hawkins/Copley/Mull Circle. They only accept kids five days a week, but your son would be young enough for the half-day program. I don't have enough good things to say about this school. I could write pages on how wonderful it is, and the woman who runs this school has been a Montessori directoress for 27 years. Not only are the kids happy there, it shows in the fact that there are rarely any illnesses, injuries, etc. For the year that my daughter was in daycare, she was bitten, pinched, pushed, exposed to head lice, pink eye, etc. The only downside is that we're stuck there. My daughter is 6 years old and doing division and multiplication - and she's so happy about it she's busting a gut. Where else could I possibly send her, now?
Good luck.
E.