These are very good replies.
I have 2 sons in an ISL BS, both hockey kids (and football and lacrosse) and we're from Minnesota. I had similar concerns about playing time for my kids, as they grew up in a great association back here and leaving meant taking a risk to an unknown.
Boy #1 is now a senior, started in 9th grade, did not repeat. He is a very good student, did the honors track at BS, and mainly went east to get better at lacrosse and prepare to play in college. He was on our A teams growing up, would have played varsity here in Minnie for a top 25 HS team. He didn't make the varsity at BS until his senior year, is playing regularly and it marks a great finish to his career. He did play all JV games and enjoyed those as well, but making the varsity and now contributing has meant a lot to him. He is headed to Tufts next year to play college lacrosse.
Son #2 decided to repeat (he would due a soph in public school) and is in 9th grade currently. He was also on the A teams here in Minnesota, and was a better player than his older brother. He would have been a top 6 forward on our HS team here. He made the varsity in his first year, and is contributing where they need him, starting on the 4th line and moving up to the second line, due in large part to injuries and his rising to the occasion (in that order). He is loving the experience.
The school he is at handles playing time like a college, you earn shifts and your performance dictates whether you get more. Obviously, injuries and school issues play a role as well. Not all the preps do it this way, and each school has different situations. Admissions plays a role as well, since the schools compete for the top kids and not all schools will admit them.
Talk to the coach about your son, and if possible, have some video if the coaches can't see him play. Most of them in the ISL will give you an honest appraisal about where they will fit in the first year. After that, it's up to your son. In our case, my elder was told that he would likely be on JV. My younger was told he would compete for time an perhaps make it int he first year, but without any guarantees.
Finally, the academic side is where the value is. So many things can happen to kids in sports and they need the discipline and fall back in the event their paths go in a different direction. BS really stands out in providing the academic preparation they need for life, with or without hockey.