I've had quite a few that would nibble when excited like that. They do tend to mouth things when super excited
. When I was working with my boy on this I would make a yarp like another pup would do if a sibling was too rough. I find that they tend to understand this better than just saying no when their so young. If they're just playing and you say "no" they might not understand why your saying no.
With Dosia, if we were playing and he nipped I'd make the loud yarp and then stop playing with him. It didnt take him to long to make the connection, he bites it hurts and I'll stop playing
it was pretty cute one time I did that and he kept trying to snuggle up to me and lick my hand, almost like he was trying to say he was sorry. It was cute.