New England is a pretty big place. Do you want something within 10 miles, 20, 30, or are you/him willing to spend overnights. Also, from what I gathered, like any business these things either specialize or are generic in nature. There are age considerations also and in that regard, some of these places will pool players in age ranges, while other limit the size and age pooling. If you’re looking for one-on-one, whoever your son has played for now is a good place to start with your fact-finding. Recommendations from that source usually comes with a “you scratch my back and I’ll scratch yours.” Also, since your son has “committed” to play college ball I assume that “commitment” means that he’s going no where else - so, if this college is within driving distance, ask the pitching coach of this college for suggestions.
Your son’s temperament, attention span and personality has a lot to do with the choice(s) made. So, on Google or some other search engine, enter your state’s name and follow that with Pitching Clinics, or Pitching Coaches, etc., and scroll down your choices.
Just be mindful of those coaches/staff that try to reinvent your son and take him totally 180 degrees from what he’s doing now. I say this because it sounds to me, from prior comments that you’ve made, that your boy is looking for enhancement, not a basic’s course. In that regard, it takes a qualified professional to know this.