it's not unnatural for pups to use their mouths in this way, but, I guess you wouldn't want it to escalate and for him to be applying pressure
Theres some tips in threads at the top of the puppies section on dealing with biting/nipping/mouthing.
Our youngest is 6 years, and occasionally she still does this, but usually only if she's excited and we're playing a game, but she does it and then licks too, it's almost like an apology for the mouthing!
When both ours were younger we did a little yelp each time they mouthed or nipped us in play to teach them to be gentle.