He has quite severe muscle atrophy, indicating that he IS in a significant level of discomfort, as he cannot use those hind legs normally, thus the muscle wastage.
If he was xrayed, I would bet that there will be significant changes to the bones in his hind legs, and soft tissue, particularly tendons and ligaments will be severely affected.
At 18 years old, particularly if he has had this problems for some time - which he obviously has, corrective shoeing will unfortunately do very little.
I would say that his condition will worsen over the years and he will become more and more crippled. The calcification in his left hock is going to be putting pressure on the small joints within the hock - hocks are one of the most delicate joints of a horse, and such severe swelling is just going to cause arthritis and other nasty conditions.
I am quite distressed to see a horse with such horrendous hind leg issues. If he was mine, he would have been put down long ago. I could not bare to watch my horse struggle like he does. It would absolutely not be cruel to put him down, I know his owner is very attached to him, but it is much kinder on the horse than letting him degrade slowly in the paddock, wasting away. Explain to the owner that the hind leg issues are going to become more severe, and the kindest option for the horse is to have him humanely put to sleep.