You may want to wait and just see how it goes. You can see on threads where people have skipped their kids, sometimes in upper grades they run into handwriting issues or concentration issues, where the child intellectually is able to do the work but emotionally has problems.
Just enjoy the ride for now.
If he is suddenly 3-5 years younger than the other kids, he may not be a very social little boy. You really need to relax and give him time to develop.