How about getting a campground space at Folsom Lake, park your car at the BLM Water Education center and walk up the bike trail (half mile or so) to the campground. This would be a good learning experience. Backpacking also requires camping skills- this allows you to work on camping skills with just a little walking. Being a little away from the car will make it more realistic. Then when you do a backpack, he at least will have some familiarity with camping. This area is open year round. And if things do not work out it is a 10 minute drive home! With a balance problem I would start out on old roads or very well groomed trails.

A suitable "real backpack" would be at Point Reyes- from parking lot to Coast Camp.

A Winter-Spring hike would be Henry Coe Park from the visitor center to Manzanita Point.

I have a granddaughter with Angleman's syndrome, and hope to take her on a simple backpack some day. Hopefully she will walk by the time she's 5 years old. Even when she walks she will have serious balance problems and lots of other issues. I remain hopeful that I can at least give her one outdoor experience in spite of this very severe disability.