Geez, Lynn, what did he do, flip over backwards and land on a rock? Sorry to hear about that.
But, you'll be glad to know that the fracture will heal up completely, assuming you give it enough time. If you try to ride him before the avulsed or displaced portions cement themselves together, he will promptly buck you off since pressure to that area will be very painful. So the amount of time you're looking at is at very minimum two months, but more like three times that. And I would try to follow up with Xrays that demonstrate a good solid callus before going back to trying to ride him.
In medical terms this is not a fracture that can usually be "reduced" or even put in a cast to stabilize. So what will be the case is that the bones will heal by "shrinkage", i.e. muscle spasm that is the normal accompaniment of a fracture will pull the fractured sections toward each other and past each other, and they will stick back together not end to end (as would be the case if you could put the bones in traction and/or in a cast), but side to side, like laying the back of your fingers of the left hand against the palm of the right hand. Thus, his back will look flatter through the affected zone. The supraspinous ligament, as well as the bursa that sits on top of each spinous process, is going to be sore as long as the bones are sore (part of why it's more like going to be six months before you can think again of riding; the bones may heal up in two months, but the bursae are going to be giving him hot, stabbing-pain sensations).
So, this animal will be out of commission for a while. The vet probably recommended stall confinement for about two months, after which they will probably tell you to hand walk, working up to a run or small pen and then pasture turnout. When he is rideable again, you'll want to spend lots and lots of time at a walk doing left and right bends, i.e. 10M half-circles, working down to 6M. Once lateral flexibility is restored, you can once again think about collection and/or normal or more advanced riding. -- Dr. Deb
Thank you so much Dr. Deb. It is so helpful to learn about the healing process for this type of injury. This was a small injury so vet never had to recommend stall rest, but a lot of time... appreciate your input.