As others have said, you have some mutually-exclusive priorities here. The only small schools that have aerospace programs are the specialty schools like Embry-Riddle that won’t have his non-engineering fields of interest.
If he wants engineering to be on the menu, I would suggest discussing a MechE or general engineering path - it is still possible to end up in the aero industry with these degrees, but he’d have a lot more options.
The only semi-affordable UC will be Merced, because it’s a WUE school. They have both MechE and a Cognitive Science major that might appeal. It’s the smallest UC campus with 8000 undergraduates. Could be an option. But can I make an argument that it would be better than WWU? Maybe. WWU is bigger and doesn’t have MechE or CogSci. And if he would like to experience a diverse, majority-nonwhite campus, Merced offers that opportunity (unlike WWU or SLO which is the whitest public U in California). What’s your estimated EFC? You won’t get need-based aid at any CA public; the question is whether WUE or the CSU sticker price are in the same range as your EFC or if you need more aid than that.
I don’t feel like SLO is the best environment for someone who needs to explore before figuring out his path. You can change majors but it’s not fluid, as you noted.
Are you open to more distant schools? CWRU comes to mind - it’s pretty small and supportive (5000 undergrads) and has an “open door policy” - no limitations on choosing/changing majors once there. MechE and Aerospace share a department, and there’s also CogSci and great neuroscience and medical-related research, based both on the academic side and at the highly-regarded med school. Also the option of Engineering Physics with a mech/aero concentration. Humanities, social sciences, and the arts are strong too… and it’s a full-need-met school. Could be worth running the NPC. It would be a little reachy but not entirely out of range - if it looked good both financially and fit-wise, applying EDII could bring it more into reach. (January 15th deadline)
URochester is another with similar attributes to Case - relatively flexible curriculum and strong in both engineering and arts/sciences. Other small full-need-met schools with engineering options that could be in range, in the eastern US, include Lafayette, Lehigh, Union, and Trinity. And WPI could be a great fit - it’s not full-need-met but there’s merit so it might work out. Not that you’ve even said he’d be open to crossing the country but there just aren’t as many smaller schools with engineering and good financial aid, on the west coast.
FWIW, U of North Dakota (which is a WUE school and very affordable) has a very highly regarded aviation program in addition to a variety of engineering majors. Not sure if actual aviation is an interest but if it is, this is one of the places to be.
Have you looked at U of Utah? He’d likely qualify for the Honors College which could “make a big school smaller”… and it could be pretty affordable between merit and either WUE or establishing residency, which they allow students to do after the first year. Strong STEM including MechE with a choice of 12 different emphases including aerospace. Great setting (both a major city and amazing outdoor recreation) and easy direct flights between SEA and SLC. UofU vs. SLO seems to be a comparison that comes up often, because they have similar strengths STEM-wise.