I think it has to be your decision, based on the needs of your son. Lots of parents want to send their kids to school early so they can become acclimated to the environment. I think the things you should ponder for helping him get a solid start are:
1. Does he feel secure when you are not with him? A babysitter can help him get that confidence so that he feels he is able to separate from you. You don't want his first real separation to be at Kindergarten,especially if it's full day. If so, that may be a battle you have to fight for weeks and it may taint his feelings toward school.
2. Does he interract well with kids his age? Does he require your help to play or can he be independent? I'm not a big fan of dropping off my kids to play and I like to see what they are doing. So, I consider letting them play at a distance and try not to interfere all the time. Correcting rude or poor behavior, but letting them test things out on their own.
3. Is he receiving some basic education... shapes, colors, counting? Can he count to at least 50? Does he recognise his numbers and letters (upper and lower)? Does he know how to spell his name (first and last)?
4. Does he have fine motor skills? Writing his name, cutting paper, coloring, drawing, stringing beads?
5. Does he follow directions? Can you tell him to do 2-3 things and he is able to complete all of them?
These are some of the things that my daughter was taught in preschool. That may help you make a more informed decision. Although you don't really have to have all of it, I think the more you can accomplish before Kindergarten, the easier the transition will be.
The most important parts are the social and emotional parts. I know a couple of people whose kids cry the whole day or large parts of the day and sometimes it continues for weeks. The best I can tell, it seems to be a lot of issue with separating from mom. Finding ways to help them see that they can be independent and succeed is so important.
Good luck to you in your decision. I am sure you will do fine. If you're considering school, keep in mind there are choices. We chose a Christian based school focused on nurturing the children emotionally, spiritually and academically. We chose 1 day a week at 3 yrs old, 3 days a week at 4 yrs old and now she's in full day Kindergarten at 5 yrs old. I feel like she got a great start in a loving and nurturing environment which fostered a love of learning and of school.
Take care,
Liz