Just to clarify, there are two ways a horn can break off: either the outer covering comes off but leaves the living core behind, or that core is broken off too.
With the first option, the horn will eventually recover itself and grow, although it will never be the same size as the other one. While it's bleeding profusely, you can use (clean) cobwebs to help the blood clot (or a proprietary blood-stop powder) then spray with purple spray. In fly season you would also treat the area with Crovect or similar, once it had scabbed.
However, if the whole horn, core and all, breaks off, you have a potentially more serious situation. The horn core contains a major artery, which can stop bleeding on its own, but applying pressure for 5 mins will help. Then, the horn core is attached to the skull and sometimes a section of skull comes away with the horn core, leaving the animal with brain exposed. If this was serious it would need to be put down, but especially for a meat boy it may well survive. We had a Jacob tup lamb who did just that - broke his horn and we could see brain pulsing away through the hole. We applied pressure and rushed to the vet, who identified the artery and left my OH holding it tightly until the bleeding stopped. The tup was then bandaged up with a wonderful purple affair diagonally across the area - he was called JimLad, or aargh JimLad until he went off for slaughter some months later. The possibility of infection to the brain was always there, and of course further damage or flies, so he did need extra care.