Formula has to be — reach out to as many coaches at as many different levels that you even might be interested in. Send them a short email and a video and see what comes back.
I would advise though against thinking that JUCO is somehow easier than D1 for recruiting. In most cases, our experience was that kids looking at JUCO were going there as a way to prepare for D1, either academically or athletically, not because they couldn't have played at a D2 or D3.
If your son is truly willing to play anywhere — he just wants to play, then I would say don't despair. My son's best friend is a 2018. Skinny as a rail but a heck of an outfielder and a solid RHP.
He went to a D2 camp with us in summer of 2017 and coach flat out told him not to commit at that time. His advice was to wait until after our high school season, which is summer ball and ends in July.
The BF worked hard, sent out video late in the fall to a number of schools and since then rejected an offer from an NAIA, and now is lining up visits to area D3s and has interest from out of state D2s.
Yes it's late for D1s, but not all schools are D1s. Up to your son if he's okay with that.