If your son is on a quality travel team with good coaching they can probably assess his potential. Step one is what level of competition conference can he play. If not the best team in the conference which ones? After deciding baseball fit consider academic, financial, social and cultural fit. By cultural fit a kid who grew up in an urban/suburban environment might not be comfortable in a more rural environment, religious versus non religious environment, etc..
Don’t worry about how large the list gets. Further research will have him ruling out some of the schools. Some of the schools won’t be interested in him.
Then he has to figure out how to get in front of his target list. What showcases and tournaments do they attend? One way to find out is email the head coach and the assistant in charge of recruiting. Express interest in the baseball program and the academics. Ask what events they attend.
If he has metrics that already would appeal to the coach add them into the email. A link to a two minute skills (not highlights) video might help. It’s a nothing to lose effort. If the coach takes a look it might help.
Players who are major D1 and better mid major D1 prospects are already in recruiting mode. For the rest of mid majors and everyone else (D2, D3, JuCo) it’s mostly about post junior year summer. But it can’t hurt to get on the radar screen the previous summer.
Never lose sight of academics. A stronger academic recruit is a safer recruit for the coach. Use baseball to gain admission to a college that might otherwise require a stronger academic record. Good luck.