K.,
You're very aware of your son's abilities and that's great! As a former school teacher, you are the kind of parent that we all want. That kind of involvement will help your son succeed for sure.
In my experience, it's always better to be the oldest in the class, especially with boys. Pre-K and Kindergarten are so much more than just learning how to cut and count. These children are learning life-long lessons; ones that get repeated throughout life.
My advice is to allow your son to go to Pre-K. He will learn a lot of crucial social skills that will help him as he advances in school, especially if he's already bright.
If you notice behavior problems, etc., then speak to the Pre-K teacher about extending lessons for your son. You can always work with him at home or get him involved in an after school program for French or another language.
Remember, your son is too young to know that he should be bored if he's mastered a concept. It's such a boost for a child to understand directions and to be the shining star of the class. Put him in Pre-K and let him shine!
One more thing. As an elementary school educator, I know that a sense of accomplishment and success in school is absolutely key for future success.
Good Luck!