Yes I know a horse that fractured its coffin bone. The horse was taken to a Veterinary School in the States, treated and eventually euthanised.
The prognosis for any type of fracture in a limb is usually not good. Personally I would look into stem cell or A cell therapy and the horse will need to be kept confined for sure. If there is a rehab center near you, after the initial healing some underwater exercise would be a good way to start getting him fit again without disrupting healing too much.
Good Luck! And take out a line of credit :P