Does he lick you and your DH or just DS?
Reprimanding him, like with a strong "No" or "uh uh" when he does it might help him to realise it's not ok. Also maybe practice getting your dog to sit/lie down next to DS and make him concentrate on you, give him lots of praise and treats for not licking DS. Putting him on a lead at first may help too. The dog has to realise that he needs to keep his distance from your son, you don't really want him thinking that DS is his 'puppy', as DS needs to be higher in the 'pack' than the dog.

Must be gross for DS though..I hate dog slobber! Lol