Congrats on your son making the next step in his journey. Depending on the program the only difference between D3's and D1's are the practice times and the flexibility offered by D3's on class interference and typically the practice start time is a little later in D3's vs D1's. At my son's program the coaching staff just expected the kids to get their work in. If they had a class conflict, let the coaches know, and figure alternative time to get the work done.
He should talk to the coaches in advance and find out what times they have their practices and try to schedule his classes around them. For STEM majors this is not usually possible, but he should at least be aware of the preferred times for practice.
D3's are really no different then D1's in the sense that the best 9 play, and best pitchers pitch. Fall is the time to make an impression on the staff and teammates. A good fall will help him break into the lineup in the spring.