Where is the kissing spines, what is the degree, has he had an xray?
Kissing spines is found in majority of working horses. I believe I was told by one Vet that a good 90%+ of working horses will have a degree of kissing spines. However it could be quite mild, or it could be career ending. Kissing spines can be the primary cause for back pain, hypersensitivity, discomfort (which can be expressed through bucking/rearing/etc.. when ridden), right up to head tossing and being 'cold backed'.
Prevention is obviously better than the cure, but you can help manage the situation in a few ways. First of all, long and low is awesome - you want to build GOOD core and back strength. If you have a horse that holds their head up quite high, for example, the spinal processes will be more compressed together which will result in more 'kissing' (bones touching), which can cause more pain. Think about this with feed as well - don't tie a haynet up too high causing him to reach upwards, keep things knee height or lower. Having the core and back strength there not only improves fitness and training, but also allows the horse to be able to hold himself correctly and cause less stress to the back and have good posture (very important).
You can do a range of exercises with him, but unless you know a) how bad the kissing spines are, and b) what the vets advice is, it's a bit tricky. Depending on how un/fit the horse is, I would start with long lining (minimal circles), or riding in straight lines long and low building up this strength before really riding him. Body work (Massage/bowen) will be great as well.