CU Boulder, no question. CU is easier to get to from California and has a top aerospace engineering program that consistently ranks among the top Nasa-funded universities in the world. CU students are also very happy and social.
To make CU feel smaller, your son could apply to the Engineering Living and Learning Community. Alternately, he could live in one of the four engineering dorms if he wants community support in a less structured way. Although I only gave you engineering-specific programs, there are numerous living and learning communities to which your son can apply.