It’s definitely a good idea to check in with the schools. Perhaps there are some opportunities (though I think you’re not in the US) to attend some on site visits/tours. Perhaps when arranging them it could be ask to see if your son could meet with a group of 9th graders who would be his peers. Possibly even a zoom panel if visits aren’t possible. Getting a feel for age difference and maturity at that point might help. Remember it isn’t just your son trying to bridge the age gap but you have to figure out, can his peers relate to a kid 2 years younger than they are, too?
I went to PS and was a year and a half ahead of my classmates though most would never know it except my closest friends and even to this day people I meet think I’m younger than I actually am. Now my DS has always been tall and has had a deep voice. He participates in a sport at a level with kids 4-5 years older than he is and many adults have thought he is 4-5 years older than he is…until he does some completely “hair-brained-whatever” which actually shows his REAL age. So yes, these kids really can assimilate with the older more mature kids, BUT there are times to realize that they are the age they are and developmentally may be completely different at times, too. As we get older those age gaps get smaller but these guys are still very much children so even 2 years can make a world of difference even if they can act more mature in certain situations.