My daughter is in Grade Three in French Immersion in Ontario. Until last week I had only wonderful things to say about French Immersion. Here are the good points:
1. Your child is going to be in a stimulating environment even if he is far ahead of other children academically. The mental gymnastics of figuring out what the teacher is talking about keeps bright cooperative minds active, even if they are learning about colours again...
2. Generally your child will be in a class with kids whose parents also care about education. In the public system that can be a real bonus.
3. Your child is only a little bit behind children who go to English school in terms of reading and writing and by Grade Three he will have caught up.
4. Your child will be taught a more "classical" and analytical way of approaching language than children in an English school. Grammar is a big part of French Immersion.
Now, here is the downside:
1. You're in Canada. Don't expect every teacher to come with a beautiful Parisian accent. Also the teachers sometimes have pretty limited English skills. You will be amazed or horrified at some of the English language letters they send home.
2. You child's friends probably will ride the bus. If you want a playdate, you'll be driving across town or into the country.
3. You are locked into the school. If Grade Two doesn't have good teachers, you can't go to another school or homeschool and then expect to go back to the same school. This is a huge issue right now at our house because my daughter's Grade Three year is just not as good as her first two years (which were completely amazing). I would say this is the second biggest drawback to French Immersion because many of my teaching friends have assured me that a good student can survive a bad year and make it up academically.
4. Here is what I see as the worst problem, and it may not be the same everywhere. In our region, the French Immersion schools offer almost no remedial teaching. If a child is struggling, he is sent back to the English schools. That can be really damaging to the child's self-esteem. It also means that some children (and parents) think that the kids at the English schools are not too bright. It can cause problems in a neighbourhood where there is already friction.
There is my mini-analysis of French Immersion. On the whole I love, love, love it, but I have to say that you will need to be patient with the school because it won't always be perfect. As well, I have seen my niece excel in the program and her brother bombed in it. It is definitely not suitable for a child who isn't crazy about school. If your son feels like there are other things than school that he'd rather spend his energy on, let him learn in English. My daughter's Grade One French Immersion teacher said she chose an English school for her own sons because she knew they wanted to spend their childhood thinking about hockey, not languages. That can be a problem for some families if they want all their children going to the same school.
As far as starting your son in Grade One, I wouldn't worry about it too much, but your school board might. Our region doesn't offer kindergarten French Immersion, so the children spend one or two years (Junior Kindergarten and Senior Kindergarten) in the English school and then switch in Grade One. Some children switch in Grade Two, but it is rare, and seems to depend a bit on the child's academic ability.
There is an organisation called "Canadian Parents for French" and they have a website that might have more information for you.