Agree with AB generally with regard to dietary help, although herbs such as comfrey *may* be helpful.
If your horse has a splint, chances are there is/was some hoof imbalance/ill health that caused it. If he was shod, remove them(I guess that's already been done) and have him *well* trimmed and *kept* that way - ie frequently, without letting hooves overgrow. Protect hooves with boots, etc when necessary, to allow *comfortable* heel first landings, to develop good hoof development & function, that will decrease concussion & aid support of everything above.