The pain CAUSED a curve in his spine, or the pain IS caused by a curve in his spine?
It sounds a little bit like it could be a muscle issue, prehaps with an underlying cause. When muscles want to protect an injured area, they go into spasm, which causes intense pain. Muscles from the spine can cause agony in the side and stomach, as well as lower back, bum and legs, depending where the area is. If, for example, he has a curved spine, a vertebrae could be aggrivating a muscle, which then spasms and leaves your DS unable to move, and in lots of pain. This will usually subside, as quickly as it came.
It could also be a pinched nerve, again caused by a similar process. Vertabrae nipping a nerve slightly, and causing spasm.
Is he on any medication? Prehaps give him some inbuprofen if he tollerates it and isn't asthmatic. You can give this regularly to bring down any inflamation, but make sure he eats with it, and don't give it for more that 48 hours regularly without making sure with your GP.
You could also try something like deep heat spray, or ibuprofen gel. Not sure what the reccomendations are for children, so check with the pharmacist before you buy it, but you may well find that even if it isn't going to be a magic cure, it will act as a bit of a placebo if you tell your son it will really help.
Have you had an xray or any kind of scans? If not, ask your GP to refer him for an xray.
Is he on any medication at all? For anything?