There are indeed lots of choices out there - and some of them will definitely give you sticker shock...
I'm on the other end of your situation now: My kids are all adult-sized teens. However they put me through the same issues as they were growing up. If your son loves riding with you, I'd spend whatever I needed to keep him happily doing it. You'll never regret the time you two spend together - or the relationship that gets built from it.
Today my children are all skilled, responsible riders - and great people who've caused me no lost sleep. I chalk much of that up to the TIME we had together doing things we enjoyed. Some families camp, boat, fish or hunt - others ride.
Don't go overboard, but spend what you need to make riding fun and not frustrating. Neither you or your son will ever regret it.