I too think French immersion would be a good fit. It's not that they have a heavier load at all in the program because the English curriculum isn't there, but the fact that he has this early basic knowledge indicates to me that his English proficiency wouldn't suffer as a result which happens in a lot of cases as the English is neglected by the curriculum. I doubt the program would serve to challenge him at all, but would likely be easier for him. I'd also push for testing so that if he is gifted, which it is to early to say at this age regardless of how early he is doing these things, he can be enriched at school which will be where he is given more challenging work tasks.