So I took my son to a new pitching coach a few years ago, a D1 pitching coach. Great guy, motivator etc. He's talking to both of us on our first visit and I mention my son is young for his grade (September b-day - most 2020's on his travel team are a full year + older). The guy gently ripped me apart (i was paying so I didn't get it full force). He said he only cares about whether a kid can compete and that fathers of kids who are young for their grade almost always tell him that fact. So I totally get why you are asking this question.
The biggest obvious and POTENTIAL issue facing the younger kids is just physical maturity. If you son turns out to be a late bloomer and you are told he would be a D1 recruit if he took a PG year its an option (just like repeating 8th grade). As you know kids develop at different times. We have a monster sophomore on our HS team who is a clear D1 prospect, he's young for the grade but physically big and quite talented. My point is there will be options down the line if your son doesn't get big and strong by this summer or fall. If he competes and is skilled age doesn't matter one bit.