Perfectly normal. When I got Moto he was all white and then he started developing the spots on his skin. He was getting more all the time. His tummy was almost covered in spots. I used to joke that when he got older he would look gray because of all the spots. Unfortunately he never got the chance to get old, so I will never know. :(
Yep, totally normal! I remember when Caney got her first spots as a puppy. Two of them popped up out of nowhere, right on the top of her back. I thought that they were oil spots that she must have gotten when running under the cars in the driveway! Thinking she was dirty, we gave her a bath... only to find that they were spots on her skin. Haha! 4 years later she's got well over 100 spots.
If you want to know what's going on though, the short and simple version is roughly:
- the white colour on boxers - that's ALL white parts, so including the white areas on flashy coloured boxers as well as those with so much white as to be mostly or completely white - is not "colour". Its LACK of colour, meaning lack of pigment. Put another way, the dog is white because his fur has no pigment.
- pigment isn't something that is fully developed at birth. In the embryonic stages of development, pigment starts to develop from a few points on the dog's body, slowly spreading out until it covers the whole. Except where the genes causing white colour prevent the spread of pigment This is one reason why flashy boxers pups almost always have bigger white areas as young puppies than they do as adults - the pigment continues to spread after birth, so more of the coat becomes coloured (or the white areas shrink, if you prefer to look at it that way round).
- not only do white areas "shrink" on flashy dogs (or the coloured areas become more extensive ) but also they tend to develop spots of pigment within all the large white areas. Those pigment spots are usually restricted to the skin only, not also colouring the hair. And that is what you're seeing on your dog - the development of spots of pigment on the skin. If you look carefully, you'll find that the spots are actually areas of black skin (versus the pink skin under the rest of his white coat), that look grey because the hair on top remains white. When the dog is wet, those spots will be clearly black. And its a process that will continue throughout his life - by the time he's middle-aged, he'll probably have a large number of spots of pigment there.