What about Donner Lake? I've only been there recently in winter, but the trails are wide and flat and it's easy to get to-straight up 80. It's right at Truckee, so if you forget anything, just a short hop into town.

http://www.parks.ca.gov/?page_id=503

Looks like you have to make reservations, but that's true of most parks these days.

I'd also suggest getting your son a pair of trekking poles to help him with his balance. If you aren't familiar with them, they are basically like telescoping ski poles. Go out enough and you will see people using them all the time. They are good for people with bad knees or backs too.

REI has a pair for kids for less than $50, but they can be had cheaper at stores like Sports Authority or Big 5. Check out the REI in Folsom, off of 50 or the one in Roseville at the big mall along 80 if you haven't been in one yet.
www.rei.com

Also, check at REI or a local bookstore for guidebooks on hiking in Northern CA. I know there are a number of them.