Usually, a bad smell around a cat's paw indicates an infection is present, so examine all four paws carefully for any signs of infection or other abnormalities. If there is a lot of hair present between the claws and pads, small cuts or abrasions might be hidden so be sure to check there as well because that's often where the infections (fungal and bacterial) are lurking. This is worth a trip to the vet just to be sure everything is okay.
Have you ever washed his paws? (So easy to say, so hard to do!) If you can, wash them with a cat shampoo or Dawn liquid dishwashing detergent (the original blue one.) It's possible he's just repeatedly stepping in something that smells bad- like, perhaps, his cat box. Sometimes, a change of litter works wonders.