Nail strengthening supplements?

I have a dog who splits his toenails on a regular basis. They open up all the way so that the quick is completely exposed. It seems to be pretty painful for him and of course it takes a long time for that quick to toughen up so it's a real nightmare for him that I'd like to help.

Anyway, I went to the vet (a new one I went to the other day that I didn't particularly care for) and asked if I could give him something like biotin to strengthen his nails and she just said she didn't know. Not helpful. I know horses and people can have biotin to strengthen nails/hooves, but I wasn't sure about dogs. Anyone have any idea if I can give him biotin, or if not, what else may help? I haven't been able to find any supplements on the market particularly for nails.

He hasn't had any diet changes, his nails are kept short, he's just done this all his life (he's 10 now). He just has naturally weak nails. :(