I have not tried the glucosamine shots nor do I know anyone who has, so I have nothing to offer as far as an opinion on them.
Is you vet averse to giving him maybe a shot of dexamethasone to help him temporarily until something can be found that works for him? Normally dogs don't do well on long-term steroids and cannot tolerate them as well as cats can, but one shot could work really well for him and actually afford him quite a bit of comfortable time. In fact, if he can be comfortable while the glucosamine shots are taking hold, that one dexamethasone might be all he needs. Steroids can sometimes be detrimental, but in a case like this, one shot could be like a magic bullet for him. Ask your vet what he or she thinks about it.