Licking is one way dog's bond with their packmates. To the dog, your DS (& the rest of the family) is in his pack. They do it to groom and show affection.

I'd talk to an animal trainer about whether or not you can stop this behaviour. It would probably take a concerted effort to train the dog. If you can't, you may have to give the dog away or come up with a way to deal with the rashes. I'd talk to a doctor about how serious the ellergy it could be (apart from the itch and redness)

Lots of luck