Oral supplements are not effective. Period. The glucosamine, PSGAG, HA etc.. molecules are simply too large to be absorbed into the horse's system through the intestinal linings.
The only way to get this stuff effectively into the horse is to inject in IM, IV or IJ. Injecting Hylarounic Acid and PSGAG once a month (IV and IM) can cost between $100-$250 /mo per horse, but it is the most effective solution.
The cheapest way to reduce the amount of inflammation in his joints is to bute him before you ride him. Along with doing this, cold hosing or icing his legs after exercise will also significantly reduce inflammation. There are other oral anti inflammatories developed that are less hard on the stomach of the horse, including Asprin, but they are more expensive, although the Asprins not by much - they are just less convenient.
Talk to your vet about options and how they could affect your horse.