Has it been a gradual process? If so, it could well be arthritis, especially if you are going into winter. Older cats certainly feel the smallest amount of cold weather!
Have you checked his claws for length? If they are quite long, they can grow into the pads and this can be painful.
My old cat, Sophie has arthritis in her front left leg quite badly, but she is still getting around fine at the moment (especially with the warmer weather coming). However she does have a large amount of calcified bone at her 'elbow' joint - when you feel her leg, the whole joint is swollen and hard.
When arthritis sets in for cats and dogs, they lose the fluid surrounding the cartilage and the bones rub against each other. Usually the diet doesn't help a whole lot (unless you are feeding a diet specifically formulated for older animals), so the bones get supplied with more calcium than they need, causing the bones to painfully calcify, and sometimes, fuse together. This is where supplements for bone and joint health come in, such as shark cartilage, green lipped mussel extract, condroitin and glucosamine (usually in the form of tablets). Metacam is an excellent anti-inflammatory to help with the pain, and is used both for long term and short term pain relief (on prescription).
Some good products we have in NZ are (these are over the counter):
Bomazeal Cat Pep (for Cats)
Bomazeal Senior (for dogs)
Sasha's Blend (both)
SF4 (cats)
MyBeau (oily mineral supplement, for both).
Let me know if you would like further info!