It could be walking dandruff bought on by stress from leaving his litter brothers and sisters, or just growing a new coat.
You could try seleen shampoo available from your vet or online, you have to leave it on them for around 5 minutes and not allow them to lick it and then start him on a course of Omega oils, like Yumega, worked a treat for my dog, who qualified for Crufts at his first show at six months, and he had terrible dandruff at 12 weeks.