As dogs age, the eardrum stiffens and hearing becomes less acute. It's possible your pug is simply ignoring you when you call, but there's a good chance that he just can't hear you. Test his hearing by sneaking up behind him and making a noise by dropping your keys or clapping your hands. If he doesn't respond, he probably has suffered partial or total hearing loss. Take him to the veterinarian to confirm the diagnosis of deafness. You'll want to make sure that it's not related to a treatable condition such as an ear infection or to neurological disease.
If your pug is deaf, he can still get along just fine using his senses of sight and smell. Make a habit of announcing your presence to him by stomping your foot when you're behind him. He'll feel the vibrations and know where you are. You can also communicate with him by using hand signals instead of verbal commands. If you're reading this while your pug is still a pup, start teaching him hand signals now; you may need them later on his life.