Thin soles are often caused by poor hoof form. Often they can be helped the most by trimming that brings the hoof back into proper balance, without toes that are run forward/long. This would be my best advice for a permanent solution: good trimming.
A good diet will also help, and biotin has been proven to help horses grow in better hoof wall. However, it does take months to see the effects.
Equinety is an amino acid supplement, and I would give that to a horse that was having muscle issues rather than hoof issues, if feeding lower quality hay. A horse needs enough protein for hoof health, but studies have not shown adding amino acids to a diet already sufficient in protein helps with hoof growth. Most of our horses get a high enough quality hay to not be deficient in protein. However, many of our horses do not have completely healthy guts, due to intermittent feeding practices, and so do not produce enough of the B vitamins such as Biotin. Horses also often have imbalances in copper and zinc which effect hoof growth, so those are worth looking into.
That being said, you can't always get thick soles in a horse that has had hoof issues and is older, so other things may be needed to help.
For example, my TB's soles have grown in thicker after having good trimming for a while, but he'll never have the ability to run barefoot over rocks. He needs boots for work unless it is on soft ground.
Can you post pictures of the horse/hooves?