For your horse I suggest the tendon and bone healing mix. This will help with some of the repair work in that joint. As a long term and deep seated problem, you will need to look at running through at least 3 buckets of this mix, but hopefully by the third you will be down to just one cup once daily (full dose is 1 cup twice a day in feed). Each bucket will last 6 weeks at full dose.
We usually find it takes a good 4 weeks to really begin seeing results and then they are quite subtle but at the six week mark generally, some good positive signs happen. During this time it is also not unusual to see a period of increased soreness as processes are stirred up slightly. This is usually around the 3-4 week mark and only lasts a couple of days to a week at the most.
The other part of the arthritis program is the Anti Inflammatory mix. The anti-inflammatory is not a pain killer, simply a pain management mix, so it does not mask the pain like Bute does. It provides relief and still allows you gauge the rate of healing well.