Keep your senior on a feed or ration balancer that is as low in starch as possible yet still caters to his elder needs
Starch will aggravate any sort of inflammation issues.
I have two mid-20's Fellas.
Except for four missing molars, arthritis in his back and mild gastric stomach ulcers the 26+ Fella is basically healthy. I rescued him nearly 20 years ago starving and with an injured vertebra.
1) If you have access to Triple Crown Senior, it would be my best recommendation. This TC link shows the carb value of all their feeds.
The Senior feed is just over 11%, which is good as long as you are not dealing with metabolic issues. Which is why my 25 yr old is not on TC SR; he has Equine Metabolic Syndrome and has to eat completely different. Triple Crown Horse Food Carbohydrate Values Formulated To Maintain Consistency
I do feed both horses alfalfa as they seem to respond well to the extra protein and amino acids (their top lines are pretty good considering
The metabolic horse gets well-soaked timothy/alfalfa cubes, while the Arab with the missing molars gets alfalfa pellets because he decided he wasn't eating the mushed up cubes anymore
He is old Arab Royalty and makes no bones about letting me know that; if he decides he doesn't like something, that's the end of the conversation and I had better come up with something else that suits his taste and is healthy for him
I am all for buying a bag of Omega-3 Horseshine (flax) as it has the proper ratio of Omega-3 to Omega-6. My horses do great on it. A 20 lb bag lasts me five weeks for three horses.
2) I would not plan on keeping your elder on Bute for too long as it can cause ulcers. It's possible, at his age, he may already have them and Bute will do more damage to his GI tract than it will to help his knees.
I do use, with great success, MedVet's 99% pure Hylarin/Boswelia on my 25 yr old with EMS. He has hock/ankle arthritis and this stuff really does work better than anything else I've tried, including Chondroitin/MSM. It's a good thing because this horse is limited in what he can have due to his metabolic issues. Hylarin-B Powder Med-Vet Pharmaceuticals (Supplements - Joint - Hyaluronic Acid)
This stuff is expensive but it's one of those things that you get what you pay for
3) Something else that does an astounding and jaw-dropping bang-up job on this horse's hock/ankle arthritis are the Hock Wraps from BackOnTrackProducts. Back on Track Horse, Equine Therapy Products
I also have B.O.T.'s Quick Wraps for another horse with torn ligaments and their Therapeutic Back Pad for this same horse who has a fractured sacrum from an accident in 2007. His loving self is a trainwreck
These products are in the "Too Good To be True" category, except they really work and are worth every penny I spent for them. I am retired and have to watch my money, so I did a lot of investigating before I bought their products.
They are not designed to stay on longer than four hours and are not at all designed for turnout. However, if your horse is out 24/7 and you have him in a small lot, you could get away with it, as long as you were around to watch him and he isn't standing in mud.
Hope this helps, and welcome!