Right in the NEWMAC, NESCAC, UAA etc. wheelhouse. My opinion is your kids grades are too good to be chasing D2's as they are not HA institutions around where you are (NE10), unless he applies to their Honors colleges. If he's had his big velocity jump, I would suspect that a year might put another 3-4mph with hard work. If you care about quality of school and want to do a PG year, be wary of some of the PG schools that are little more than sport farms that won't help at all with getting into higher academic schools. But expect to Pay @70K for that PG year, probably more than any scholarship will make up for, and there is no guarantee that gets you mid-major D1 or he ever sees the field. Some suggestions on PG (Exeter, Andover, Cushing, Avon, Cantebury)
I do know a Lefty going into Sr year committed D2 in your area sitting 80mph after a strong jr year on a very good D1 HS team ( a league with multiple d1 talent). So D2 is within grasp if so desired, D1 might be a stretch even with a re-class or PG and significant increase in velocity, even with hard work, is it worth the risk/reward/expense?
Don't let him have Senior-Itis, A 3.9 can drop very quickly.
Best of Luck