"He's also been admitted to several D1 schools that offered no support in admissions but are giving him the basic "you can walk on" email."
Most of the D1s have locked up their recruiting and spent the available scholarship dollars at this point for the 2016s and probably most 2017s, too. However, there are ~7+ players on each team that are not getting athletic scholarship dollars, and are vying for a spot. If your son gets his pitching numbers up, there could be an opportunity. Many D3s recruit during the high school senior year to plug holes in their roster
Your travel coach and/or high school coach should go to bat for him with any contacts they may have during the coming season if he is performing at the level of the schools he targets.
The draft opens up roster spots each year, too, and all coaches want pitchers that can throw 90+.
Several years ago from keewartson's high school, I know of a player that got "recruited" to a D3 in May of his senior year after the coach saw him play a new position and another to an ACC school in June of senior year after the university lost their catcher in the draft.