Hi I'm late to this post but just to say the areas he is infected in are the most likely ones to be affected by mites. They go for areas where the skin is sensitive and blood is close to the surface. Mind u I don't think ur pic looked like mites unless it's under the skin. There is a type of mite called cheyletiella which is nicknamed walking dandruff and is clearly visible as white bits on the surface of the skin but if you watched closely the bits would move. there is info on that sort of mite here.
Hamster mites, and ticks
Hope whatever it is clears up soon... Btw mites r easily treated with ivermectin and a thorough dinsinfect of everything so if it does turn out to be something like that don't panic. I'd tell the vet it's spread to his boy bits and check before u out the cream on but sounds like it would be a good plan.