Firstly, it's nothing to do with hoof colour. There is no strength inherent to the pigment, which is only in the outer layer anyway. Pics of his hooves - check out signature link - would be helpful if you want specific opinions/suggestions on his feet. Diet & environment are 2 huge factors in hoof health/strength. So providing us with info on these factors & his lifestyle would help. I see you supp with biotin. That can indeed be helpful for hoof growth, but there are a range of other nutrients that are important & often deficient/imbalanced, so I'd look into that too. Feedxl.com is a good source for nutritional/diet info & help.
Stone bruises are common with horses - shod or bare - worked over hard/rocky ground if they have thin soles. Bruising can last weeks & can also develop into abscesses. Shoes won't help him, aside from palliatively, without pads at least & I believe hoof boots are generally a far better option for the horse.
If he lives in a wet environment, his hooves will be softer. While chemical hardeners can definitely harden soles, what is needed is for them to *grow thicker* to provide adequate protection. I wouldn't use chemicals, but if in a wet environment, I'd endeavor to provide at least some dry ground for him to hang out on. Soaking his feet regularly in a strong saline solution can help dry them out too.