The most important piece of advice I can give is to believe in your son and to believe that his goals are obtainable. Provided there is a baseline of intelligence / aptitude, he might be better prepared to succeed in engineering than you think. Some kids are just late bloomers and there is a wide variance in the competitiveness and difficulty of grading at various high schools. One high school’s B in physics might be another high school’s A or even C. There is also an incredible variance in how well different high schools prepare their students for college. A kid with 3.4 GPA from a solid high school with academic rigor might actually be far more prepared to study engineering than someone from another high school that has achieved a 3.9 GPA with less rigor. Test scores and one off AP scores are not the be all and end all of everything but in aggregate they can give strong insight into potential.
Unfortunately, for a smart kid with a lower but upwardly trending GPA, there might be a strong disconnect between what colleges they can handle the work at and where they can actually get admitted to.
I think Cal Poly and Colorado School of Mines are great schools and should still be applied to but might be tough admits given the GPA. For Cal Poly I would defer to the CA experts on this board. My own son squeaked into (but did not attend) Colorado School of Mines with a 3.3 or 3.4 GPA but he is an URM and had a 34 ACT. There is no doubt in my mind based on his grades at his current university that he could have handled the work and thrived there. We loved RIT when we visited - very nice modern classrooms, labs and facilities. I could be wrong but I think I remember reading that ME is a very difficult major to get into at RIT. We toured Clarkson and liked it but could not get over the middle of nowhere location. I personally think some of the lesser known state flagships or regional state schools are the way to go in this situation provided he can be a direct admit or there is a realistically obtainable pathway into the major.
To me, going where you can get into and stay in the major is key. It is the major not the school that will largely determine future earnings.