So, the short answer to your questions ... yes, there are ways to get on the radar of those schools. Any combination of -
-If your son has credible reference like an established pitching coach, travel coach or other that can advocate and would be willing to reach out to targeted school/s, that can be a start.
-Sending a good video demonstrating the desired skill set and measurables along with compelling reason he is interested in that specific school.
-Identifying the key events where the school/s will be actively recruiting and attending those.
-Identifying and joining strong travel programs that will be attending events in front of those targeted schools (of course, player has to have the desired skill set that he would get playing time with said organization).
-attending a prospect camp at targeted school only after initial interest has been established and there has been agreement that player would get a good look.
Also, yes, generally, a player with a D1 skillset but not one that stands out exceptionally will likely have more opportunities at those schools that are both in the northern climates and are traditionally bottom half finishers in their respective conferences, because those schools of course have more difficulty attracting top shelf recruits.
But I would also add to check the logic. I don't know how much of this applies to your player but... you describe a player who can't crack the roster at top P5 schools. So, such a player isn't likely to get a ton of money from those next tier schools either. Do you really want the player to travel far from home to a much more harsh climate, pay an extra $15-30K per year for out-of-state or private tuition, play on a losing team, have a more challenging and expensive recruiting path with less assurances likely, etc., so he can say he played P5 D1 instead of other D1 or D2 (or ??) where he can perhaps play with a title-contending program offering more athletic $, be more likely to contribute significantly, have lesser costs otherwise and be closer to home where he and his family can enjoy the benefits of being reasonably nearby?
PS - took a while to finish my post... I see that many of the same things were already covered.