The Notre Dame opportunity sounds fantastic from an academic standpoint. If he were my son, I would say that would decide it if he gets it, then he would just focus on trying to make the team as a Walk-on. It certainly would help your son to contact the HC and let them know he is interested in one of the 8 Walk On spots and with his grades and potential for success at ND, they will certainly give him a look (no doubt all of the scholarship spots are filled now). They want a strong team GPA and your son being part of the 35 would likely help that a lot. As others have noted, Walk On is a tough choice and limits his chances of playing, but getting a full ride to ND and sitting on the bench of a D1 ACC is not a bad gig.
If you don't want to put all of your eggs in that basket, then he should rush to get in some applications at some academic D3s. I think the deadline for Pomona-Pitzer and Claremont McKenna have already passed, but Occidental and some others on the West Coast are still open for a few days. Better to get them in and then sort it all out than miss the deadline and then be left with no fallback if the ND option does not materialize.