First of all, it’s early. Your son will be maturing quite a bit between now and May 1, 2020. Some of mine changed their minds. Even those who did not had streaks and times of ambivalence. So this is not necessarily going to be THE school for him
Secondly, absolutely apply. Cal Poly is a wonderful school from all that I’ve heard about it. Unless he applies, it won’t be an option for him. So put it on the list.
Thirdly, as long as he has one sure thing school, where you are as sure as can be that he will be accepted and can get his engineering degree, apply with wild abandon.
Fourth, have a variety of cost options available so you and he can value rate his choices in May. No reason you have to take the least expensive or best value. But go through those paces.
Fifth, understand the drawbacks of going to an OOS public school that doesn’t have a lot of OOS students. There are a lot of built in presumptions in schools and other places that are geared to the vast majority. Some of those things, kids know through conventional wisdom gained from time spent in a state. Talking to instate classmates, teachers, parents etc. when you are outside that circle, you can miss out if not aware and proactive to finding out these things , often opportunities , reputations, subtleties. Also you are not going to have as many degrees of connections with students. My son particularly noted this. His cousin was at same school and knew ever so many people and even more with connections. My son did not. It can feel lonely at times, especially if not social and not used to that situation, and sensitive to it
It appears to me, that you are missing the fact that OOS students do not get extra points for honors or advanced courses at Cal Poly . Only specifically designated AP courses I remember kids frustrated with this issue from some OOS school or other, coming from a very rigorous prep school that did not designate ANY of their courses as AP, or honors , for that matter. Anyone could take an AP exam, and the courses covered the material with additional test prep sessions offered before the exam. But colleges that were looking for rigor, had to know the school by reputation and school profile. A lot of state universities failed in this regard for all their touring that they looked at each applicant holistically as well. Just not possible at large universities that have to evaluate so many applications in such a short time.
Great that your son has found things he knows he likes in a school!