Visit the schools, they are all good but might not be a good fit for your child.
If you haven't, I'd recommend having your student take the SSAT (or at least practice SSAT's) to get a general sense of where your student will be admissable. If the grades and the scores aren't really good, I wouldn't risk getting your kid all jazzed about Groton or St. Mark's. Plus, the admissions process (applications, essays, recommendations, interviews, etc.) can be time-consuming and energy-draining -- I'd recommend not wasting your time on schools where you don't have a reasonable likelihood of admission. (And yes, if your kid is a really good hockey player, the admissions process can be a bit more flexible. But, depending on the school, it's not that much more flexible.)
As others who have gone through the process will tell you: don't focus on the hockey too much. Find the right cultural and academic fit. You will likely approach each school with some sort of preconception, but you'll leave with a feel for whether it's the right kind of place for your kid. Follow your gut on that.
If you had your choice where would you send your son?
My oldest goes to Rivers now in the middle school and it's been a great fit thus far. The teachers know everyone individually, every kid has to do a sport every season and play an instrument- nobody can hide and just coast. I prefer the non boarding option as I'm not sure my kid would have the maturity to handle it that young.
Best advice is to go to open houses and see what's a fit. Do you want an all boys school or coed? Do you want boarding or no? Does your kid under no circumstances want to wear a jacket and tie every day? Do you not want any religious affiliation at the school? Do you want an old school lecture style chalkboard classroom experience or do you want a new school flipped classroom with a collaborative team based approach to learning using current digital tools? Do you want your kid rubbing elbows with the kids of old money railroad barons or children of the Chinese politburo? Do you need aid or are you a full payer? Certain schools have very large endowments so even if you make bank you often get something.
But what's more important is to not think of these schools as hockey levers but life levers! These schools open up more options than you can get even from top public high schools. Early birds get the worm but late bird owns the worm factory- be like late bird.