I would get with your vet and a few farriers, hopefully find a barefoot one also and see what all of them have to say about his knees. Sometimes shoes can help correct minor problems with legs but generally barefoot is a better choice and helps out more in the long run. See what everyones opinions are and you have to make your final decision from there.
I would not worry about riding him until he is quite a bit older, see what your vet and farrier says about his legs and ask them about riding him. They will be able to offer the best advice after seeing the horse.
You know how to make a miniature horse even smaller? Leave them in the dryer a little longer!
"Don't ever regret something that once made you smile"