Here is a different way to assess this because you seem like an honest down to earth parent. If you think his hockey career will end at high school, then where will he have the most fun and the best experience? FS u18 and u16 teams usually play in front of a few parents during weekends when his HS buddies are at school events. HS plays in front of a student body and is usually covered by local media. There are other benefits of playing sports for your high school that most of us remember. The FS is likely to be the better hockey and instruction but maybe HS is the best way to finish the career? You will also save a few bucks for college
There are plenty of benefits to either choice. I think if you look at the thread called "A good coach", you will find most of the pros & cons discussed ad nauseum with respect for FS vs HS.