GuitarStv, sorry you have to go through this. Reading your post brought back a lot of memories, not my kids, but mine, even after 30+ years. I was the youngest in class (and grade) from age 3 till 18.
I still remember vividly how I was "misbehaving" and punished and shamed in front of the whole class in pre-K so harshly, and for so many times, but I wasn't able to tell my parents properly. The punishment somehow didn't stick, I guess that's the impulse control issue. My behavior was so "bad" the Pre-K teachers repeatedly told my parents to pull me out, they did and they got me to start actual school earlier through some back channeling favors. I was at the top of my class, but always got timeout because I constantly wanted to talk to my classmates when the teaching was ongoing, because I was bored. Elementary school went by OK but middle school was disastrous, I remember I was talking to a classmate, and being called out by the teacher, I smiled not knowing how to behave, and the teacher scolded me as "shameless" for smiling and boy did I tank that class, I couldn't focus when that teacher was a round, I was afraid and angry.
Growing up I was being told or overheard grownups talking that I was younger than the others, too immature, etc. So I allowed myself to behave like a little kid and without enough self discipline. That mindset stuck with me for a very long time. I didn't have a lot of friends till later on, I think partially because my classmates see me as "immature" too.
Come to think of it. The only "gain", if you can call it that, is that I graduated and started working FT a bit earlier than most, but the downside was plentiful. I really wished I had an extra year, had more friends, could express things more clearly with more mature eyes and mindset, could exercise more self discipline. It could have made a happier childhood to say the least.
Do whatever you need to and help him grow up right, and happily, don't ever let anyone plant the idea in his head that he is immature, or bad, or whatever than his peers. If it takes a year off, so be it. Reading your post brought back a lot of feelings I thought I had long forgot.