I think that your son my be way too young to start preschool. Your son has a late birthday, so with that being said, he will be 3 yrs. going on 4 when he is ready and able to be accepted into a program. It is never too early to start looking though... . My son has a late september birthday and will probably be one of the oldest kids in his class, as well. He is 3 1/2 and will start preschool this fall, and then will turn 4 shortly there after. He will be doing 2 yrs. of preschool since he will miss the Kindergarten cut-off. Which will be good for him...more socialization in a controlled and stimulating environment at a more appropriate age.
Preschools, opposed to daycares, should have daily lesson plans to guide them through their day of play. Yes, children do learn best thru play, but, there needs to be a purpose and some guidance for their learning. So look for that!
Your son is still at the "parallel play" stage at the age of 2. Playing and learning with a group of other small children is often difficult. 2 year olds don't really interact yet with their peers...this starts to happen at 3. Their imaginations take off and they usually start making friends and tend to want to play together more. Sharing is never easy, but it does get easier during the preschool years as well.
Hope this helps in your decision making...