As far as where to start, use the search bar and look up what you want to know. There is more information on D3s here than there is anywhere else. This place is arguably more valuable for D3 recruiting than it is for D1s, and it is very valuable for D1s.
If your son is on a team good enough to be playing in Jupiter it may be worth exploring the juco route. Especially with his velo being as high as it is. You already mentioned D1s not wanting to recruit 17/18 year old kids when they can recruit 20 year old men. If you go the juco route your son can be the 20 year old man they pick up.
I know it's not for everybody but realistically if you're not going to a HA D3, there really isn't any sense in paying all that money to go to an average to below average D3, unless it's in state of course. Juco makes more sense fiscally at that point.
As far as Jupiter. That's really for pro eyes mostly, it will probably be a waste in terms of recruiting effort and return on investment but it might be the highest level of competition your son ever plays. There will be multiple first round picks there.