I'm afraid he still looks really thin to me. I'd be giving him hemp oil or camelina oil, and either a senior feed or a similar type product. Our Harley is 21 and had a harder time gaining weight over the winter, but he still looks much fuller than Montana (sorry, I know you're doing your best, really). Harley is my daughter's horse and she will be competing with him in dressage and low fences again this summer. He is ridden about 5 times a week by her (a 100 lb 15 yr old in a light cc saddle) and has tons of energy, but I still feel he needs extra calories (even moreso with that schedule), so I've just put him on Elite Equine Evolve by Elite Three. Also, Harley was diagnosed with arthritis in the hocks last summer, but the vet said we should keep working him. We put him on Previcox (Equiox) for a few months, but when we ran out and couldn't get more because of the pandemic, we realized he wasn't moving differently without it. So we are now assuming his hocks have fully fused and he isn't in pain. We aren't planning on putting him back on the Previcox unless he shows us he's hurting. So even arthritis isn't necessarily a reason to stop riding a horse. Movement helps them stay healthy much longer.
I don't think you need to stop riding him since he appears to really enjoy it, and is full of life in that wonderful photo, but I do think he needs calories asap. If he is gaining weight, then keep at it, you're doing something right! If not, definitely add something else. Hemp oil and camelina oil are anti-inflammatory so those are generally considered better than other oils.
I think that with the right diet, he can still keep going on for a long time.