I would not be concerned. Puppies will nip, and although he is 18 months, he still has some puppy in him. Even our cat, Jackson, some times nip us, but not that hard. If you want to break him of this habit, when he nips, tell him sternly "No", and do not touch or pet until he stops. In most cases, this will let him know this is not approved behavior, and you do not like when he does this. Dogs generally want to please, so he will eventually learn not to nip. Good Luck !!

Oh, BTW, welcome to Pet Talk !!! A nice group of people come here, and they're very helpful.