Participation in the SoCal Scout league along with acceptance and attendance at the better Stanford camp (and/or a PG event) will give you both a real good idea where he stacks up. I've heard that statement about "there's a college for every player" before and I don't really agree. Realistically, the average player will finds it very surprising how competitive even most D3's and NAIA's are. Many decent/average HS players (not saying your son is), have to be willing to travel to the far reaches to find the armpit programs in order to get a chance to play. Also, there are many decent players who are not as academically gifted as your son who have to scratch off D3's and NAIA's because they don't qualify for academic $$ and can't afford to pay the high tuition.
Another point of reality - a FB that sits low 70's will not get much of a PG rating or attention otherwise, regardless of how effective he is in HS. Here is something else he will have to deal with as he works to improve his velo - HS hitters tend to struggle with P's that throw above OR below the typical HS velo range, say 75-83. So, once he improves to 75 or so, he may find himself LESS effective. He will have to rely even more on movement, location and mixing smartly.
You included D2 in your question. Pull up some D2 schools in CA. Do some cross-checking of the roster names. You will be hard-pressed to find many players who were not at least HS 1st team All-League if not All-State caliber. Lots of really good players in CA for the colleges to choose from. Not trying to discourage, just letting you know what you are up against and what he'll need to work at.