FS offers a huge advantage over MOST MIAA programs. There are exceptions both ways. But, simply based on his receiving continuous instruction and consistent practice time for six months instead of three will lead to greater development.
YMMV, but most half season programs rarely practice, and most HS teams are focused on beating the teams from the three surrounding towns for bragging rights, not player development.
And, the exodus of talent from the HS teams in just the past few years means he will be playing against lower overall levels of competition. He may be fine with that, in exchange for the experience of playing in front of a crowd and the notoriety among his classmates. Most FS kids are playing for development (with some being a bit unrealistic about what their hockey future is after HS).
Now, there are bad FS programs out there, too, so these are not universal truths.
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.
Just want to say thank you for the question and the helpful and balanced answers. In the same boat here and really appreciate the feedback.
Go watch EHF second level or e9 midget hockey this fall. These are your HS players. That will tell you what to do!