Well, the term Heterochromia refers to a condition in which two or more diferrent colors are presesnt in the same eye especially in the iris. Heterochromia can be caused by many genetic as well as physiological factors like mosaicism, chimerism, disease etc. In the case of your dog, yes, It's a chimerism for sure but I'm not so sure about Heterochromia as I can't see his eyes clearly but It's very possible to have both Heterochromia and Chimerism all together.