Pics would be lovely.
I'd talk to the farrier about doing glue-on shoes. It allows the hoof to expand and function normally (shoes tend to reduce the natural expansion), reduces shock from walking (which can worsen cracks), and can even help with moisture balance since most glue-ons have a sole. Check out Renegades or Easyboot glue-ons.
I personally would ditch the shoes. Boots seem like the better option. Through a big roll on the toe, let him prance around a level field 24/7, and I think he'd do just fine.
Might want to think about your supplement choices. 20mg per dose is at the higher end of the biotin-dosage spectrum. 100mg is not something I've ever heard of before. I'd think you'd get the same results with just 20mg. Do remember that feeding a biotin supplement can cause the body to reduce its own biotin manufacturing, meaning that if you take him off the supplement, his hooves go downhill because his body isn't making biotin for itself anymore.
So I'd go for the lowest dose that works for him, and I'd give it a few months off in the winter to let his body pick up on making its own biotin.
Check out smartpakequine.com for more hoof supplement options. They have biotin alongside zinc and other ingredients that really seem to help, and it's cheaper than what you're paying.
By us the ground is super dry, so everyone's hooves are super dry. Try wetting the area around the water trough so he stands in mud a few times a day.