AD2018,
Also from California with boys who've played / are playing D1 and D3.
D1 players generally get placed by their coaches in Summer Leagues the prior Fall, i.e., way before Spring playing time.
In my experience, D3 players may or may not get coach assistance / placement for Summer Leagues. And many D3 players choose to pursue work, internships, etc., instead of or in addition to playing in the Summer.
And pitchers have a lot more options than position players and can get placed later...all the way through the times Summer Leagues start (thank you HokieOne!). D3 position players are lower on the totem pole unfortunately...and I have one of those too.
I recommend that your son talk to the college coaches who are recruiting him to understand how they do Summer League placement. And talk to some current and former players. Quite an eye opener.
And then once a college Freshman, he engages his coach in the Fall to discuss placement. If the coach makes a placement that works for your son and family, then great! If not, then the player can engage Summer Leagues and teams directly. Typically there are web forms for prospective players. And most leagues will have a GM and Head Coach directory so that he can email.
My oldest had "premier" league placement this Summer but had way too many innings for a 70+ game schedule and loooong travel times between games. So he said no to that. He had to take a Summer class and then was very blessed to find a great opportunity in an awesome league that played 42 games. My youngest got no coach assistance (none of his college teammates did either) so he reached out to a league and was selected for a spot.
There are a lot of leagues. You can peruse rosters to get a sense of where the talent comes from. The California Collegiate League does have D3 players and is a very good league. But there are a lot of leagues that could work. Some considerations: roster size, playing time, travel distances between games, number of games, host family vs. paid for housing vs. living at home, georgraphic location, need to work/intern, etc. etc. There are some leagues with an enormous number of games and very long travel distances between locations. There are some leagues where a player could work or intern in the morning and play at night. There are leagues that have only 20 or so games and cost $2,000.