Apple Cider Vinegar 50:50 with water works well.
ACV works sometimes because fungal infections don't tend to like acid. If it's something that doesn't mind acid, or a deep, closed infection where it can't get in, it won't help. I tend to use ACV on minor seedy or thrush, or as a preventative measure in less than ideal environment, but I'd go something broader spectrum in a case such as this.
Has anyone had any success with general hoof supplements? Like stuff from smartpak? I don't like supplementing unless it's for a reason but biotin seems popular for coat and hoof but does it really work?
I do think diet & nutrition are vital components of healthy horses(including hooves of course). As pointed out, I suspect the 'symptoms' are also diet/nutrition related, so yes, if you don't already supp the horse to ensure balanced nutrition, that will probably be a good move. The thing is, adding particular nutrients 'willy nilly' can be an unhelpful waste of money. So in order to choose the *right* supps the first thing is to evaluate his current diet so you can see what's missing. Including a pasture/hay analysis will give you a far more accurate idea, but failing that, your vet or others in your area may be able to tell you more about the soil/pasture types for a reasonable idea. I find FeedXL.com to be a really helpful program/service for working out nutritional balance & what products will fit your particular horse's 'bill'. Not to mention having qualified nutritionists to learn from, ask questions of, etc.
Smartpak does seem to be one of the better pelleted type supps available(important to me because very low dose pelleted good for my fatties who don't get other hard feed), but Smartpak isn't available over here, so I haven't used it. Biotin is one vitamin that has been studied & found to help hooves grow faster, apparently even if fed in excess of 'balanced' guidelines. It is readily available in green feed, so if your horse has adequate grazing or is fed some alfalfa or such it probably won't be actually deficient in his diet though. Mg is one that more research has suggested the 'accepted' ratios may leave horses(& people) quite deficient & oversupplied in calcium. It is also affected by stress - be that physical or mental - and high carb diets. **I'm no nutritionist & don't take my word for it, but well worth looking into IMO.