you can also talk with a smartpak customer service rep. They are super helpful.
I would probably just put him on a basic supplement for now and maybe bump it up if you started competeting or training really heavily.
you can also try more natural ways to keep his joints healthy. Carrots and yams are very good for joints. horses can have up to 5 lbs of carrots a day and you can give a him half a yam a day as well. If you were to do both, I would give him half a yam and maybe 2 lbs of carrots.
"Take the time it takes, so that it takes less time."
"Expect a lot, accept a little, reward often."