It's pretty normal
What you've got to figure here is that the white areas aren't colour. They're the exact opposite - areas of NO colour. That is, areas of where pigmentation did not develop. Pigment is something that starts developing in utero, but that is far from complete at birth. It starts out from a few specific spots on the dog and spreads from there. So it is perfectly normal, even near-inevitable, that the extent of white on baby puppies is going to "shrink" as they get older. Or the dog grows and the white doesn't, if you prefer to look at it that way.
Now, I know you're not talking about a baby pup here! But irrespective, it's still normal for pigment to continue to develop as the dog ages. On whites and on dogs with large areas of flash on the chest and belly, for example, as the dog gets older we usually see a lot of pigment spots start to develop on the skin (like freckles). The pigment there isn't usually "strong" enough to also colour the hair, but it's continued development nonetheless. All normal
In your guy's case, the flash on one side of his face was always far less pronounced than on the other side. And that's where further development of his pigment has gradually eroded the white.
Anyway, it is all perfectly normal. If his facial white had been even, then probably you wouldn't really have noticed it shrinking much (even though it may still have shrunk) because it would do so evenly. But because one side had stronger pigment in the first place, it is just more obvious, I think. Either way, it is no problem at all and is a perfectly normal and natural occurance