We recently introduced a puppy to our family having already having our boy. Logan is a very submissive, gentle giant who wouldn't hurt a fly and did end up looking rather scabby with puppy bites and scratches

but he just wouldn't tell her off so we did have to start intervening very slightly. When things were getting out of hand and it was obvious Logan wasn't going to put Piper in her place we started having "quiet time" where we put the pup to bed to calm her down, every time she'd be fast asleep in about 30 seconds but it seemed to really work.
Piper is now 18 weeks old and as she's much bigger Logan will now let her know if she's being too much. I think he was just frightened of breaking her when she was really tiny, but now he know's she's a bruiser and can take the odd telling off from him.
They have a fantastic relationship, still play fight, but doesn't seem to be constant mouthing anymore.
So the "time out" way definitely worked for us.
Good luck with everything, I'm sure they will sort it all out themselves
