It's highly unlikely your son will get anything. When I had a dog I'd kiss her on the head or the snout all the time. Kissing the dog on the mouth is nasty but your 2 year old doesn't know any better so it's kind of cute. Dogs don't share many illnesses with us so there isn't much call for alarm, especially if the animal lives inside and you all are healthy, he's just not going to have any opportunity to grab any illness to share with your son. Laugh about it and don't worry! Your son will be fine, and it's great he loves that dog.