If you are unable to keep the horse sound at the current level of work and the additional things you're doing aren't enough to keep him sound, then you have very little hope to get him doing any more. To expect without further "out-put" is a bit ridiculous in my mind. So if you don't have the funds to do more to get the horse sound for light work - or you're not prepared to dump more money at it IN HOPES of getting the horse sound, then I would suggest you retire the horse, or drop it down to a level of work that is comfortable for the animal. I'm going to add that correct ridden work, where the horse works over the back is only a good thing with a horse with back problems (assuming the saddle fits & the rider is a good rider). It will have the horse lift his back, engaged his abdominal muscles which will only help the horse have less strain on it's back. BUT.....a poorly ridden horse, or a poorly fitting saddle is going to cause more harm than good. So if you & your cousin aren't riders capable of riding in such a manner, then I suggest you call it & retire the horse, as that is in the horses best interest.