I think that just as with humans, age-related hearing loss starts at different ages. I had a great aunt who could still hear a pin drop across the room at 96, while Dave's mother was wearing hearing aides at 40. We had a white cat (they are prone to deafness anyway) who was completely deaf for the last several years of her (long!) life. Our current (also white) cat is 14 1/2, and her hearing is FINE. Animals adapt to deafness even more easily than they do to loss of sight. (and they adapt to that fine too)
And with Djangos on-going health problems, either his chronic illness OR the meds he finds himself on for it could affect his hearing in the long run. (you can ask the vet about that)
In terms of what to do about it, if I were you, rather than "just" talking louder, which could be only a temporary fix if he is really going to lose his hearing completely at some point, I would start to transition him to hand signals.
The way to switch a dog from one signal to another is to use the new signal, immediately followed by the signal they know. Since dogs tend to anticipate anyway, they eventually start responding to the new signal because they know that the other signal is coming.
Obviously, this doesn't work with a deaf animal who is sleeping, or in another part of the house. That was our biggest problem with Angel… she'd fall asleep in the back of a closet (cats sleep in the strangest places!!!) and we'd have a nervous breakdown when we couldn't find her, thinking our deaf cat had gotten out of the house!