I have a navicular horse who is doing just fine at age 20. She is kept barefoot.
I would take a close look at his hoofs. Compare the frog size/shape to a normal bare-foot horse's frog. Is his hoof contracted?
The problem with shoes is they can cause hoof contraction. When the hoof contracts it inhibits shock absorption and can cause navicular.
May 2000 California Thoroughbred
Getting the hoof to expand, should help the horse stay comfortable.
If you cannot afford shoes, you could try hoof boots. If the foot is contracted, the foot really needs a cushion to help with shock absorption. Hoof boots are better than shoes in that regard.
Here is a long article but well worth it.
DIGGING FOR THE TRUTH ABOUT NAVI
New Page 3
Unfortunately my mare's condition was diagnosed too late. The contraction was irreversible. I often wonder if it was caught earlier if we could have gotten her hoof to expand. By the time I learned farrier work, it was years too late.
It is possible to get the horse stabilized at least. My mare has had "navicular" for 13 years now! I don't think it has really progressed. Her lameness has not gotten worse so I have not done more X rays to check.