No direct experience, but a fair amount of observation.
I see you're in CA. You can do a lot of nosing around school websites to see where kids have gone. This link is useful because it will take you to every Juco program in the state.
You can also look at the rosters of the 4 year schools where you would like to see your kid after a couple of years and see if they include many Juco transfers. The top privates listed by RJM will have very few transfers. But there are more at high-academic publics.
The Juco closest to me has a grad who is a senior starter at Michigan. High academic P5 program, though cold. This Juco has 2 recent Cal commits, as well as at least one recent transfer to another UC (though one of those went pro instead)
UC's can and do take Juco transfers after Freshman year. But as a practical matter it sounds like that wouldn't apply in your son's case, as these are kids who are committed in the fall before they have played a single JuCo game. Typically they are kids who need to get their academic act together, or kids who decommitted from other programs late.
Bottom line I think is three things. 1 - California residents are fortunate in that the UC system has a number of Juco transfers built in. If a student does his job in the Juco classroom, there is room for him in at a UC. 2 - players who really excell on the field will earn 4 year opportunities, high academic and not. 3 - even assuming a kid is 100% qualified academically, getting a slot at a high academic D1 is really, really difficult, regardless of whether you go Juco first. You have to get in the conversation before worrying about things like conference, weather and standings.