Hoof strength (alongside all other horse traits) is down to genetics so there may not be all that much you can do. To give you the best chance though you should combine a good diet (hay, grass and a good quality feed) with supplements. Biotin is one of the few supplement with scientific research to back up its claims to improve both hoof and hair health. It is best in combination with a whole range of other supplements such as zinc, methionine, lysine and cystine, fatty acids, manganese, copper, and vitamin C.

There are a whole range of off the shelf supplements available so I would recommend looking at what they contain and picking on that contains most if not all of the above. It will take several months for any supplements to show a marked improvement due to the rate of growth of hoofs and hair.