My son now 12 started at 5. I think it is an individual decision based on the mental & physical maturity of the child. It soulds like you have some good reasons to start your son. The other comments by those with similar experience seem valid. We had had a few kids that started at 4 over the years. Some were ready
and some were not. In fact, I've seen some at 6 or 7 that were not ready. My experience suggests to avise you to not push too hard. You don't want to create burn-out in future years. We have experienced an little of this.

Again, it's an individual decision.
Best of luck with your youngster!