He's just being a mouthy, attention-seeking puppy.

Teach him that nipping will only remove your attention and end play. Thus, when you are on the couch and he's mouthing you to play, get up and walk out of the room. Close the door on him. It seems a little drastic, but playful puppies often enjoy "bad" attention (i.e. you yelling "No!" at him) so it's best to just completely remove yourself from the situation. When you walk back into the room, calmly go to the couch. If he mouths you again, repeat. As soon as you can walk back in and he's sleeping on his own bed or calmly waiting for you, you can praise him. Same routine when he's outside. If he's mouthing you to play, walk back into the house and remove yourself from his presence. Come back out into the yard. If he mouths you, leave again. If he's not mouthing you and is instead sitting or waiting calmly, praise and then play. It'd also be a good time to enforce Nothing in Life is Free (if dog wants something, dog must perform for it - like "Sit" before you can eat, "Down" before I give you attention, etc.).