My nephews (twins) have a Sept 19 birthday and the cut off was Sept 15, but they went to K at 4 anyway. One is happily in college. The other decided against college and his first year out of hs he was the asst hockey coach for the hs - with several of the seniors being older than him. Now he just completed the EMT certification and some jurisdictions won’t hire him because he isn’t 21 yet (and won’t be till next Sept, after two more hiring and training classes. Will the world end? No but it is a limitation.
I think it won’t matter in a few years to OP’s son but right now he’s still unhappy about HIS decision as a freshman in college to not study harder. Was that because of immaturity? Probably, but maybe he would have screwed up in hs because he was bored at not being challenged if he had started K a year later. It’s not possible to know where the road not taken would have ended.