my dog is biting my other dog’s neck

he is now 1 year and 4 months. he’s a standard poodle, shes a spaniel x poodle to give a size difference. he only nibbles her neck in play, little low growls, pulls away then goes back to nibbling. i would call it nibbles because he doesn’t actually bite down on her neck and stay there. but, my problem is he sometimes ends up hurting her. when i’ve told him to stop he does but then she runs back to him and he does it more, and it is very tiring. i need some feedback before i try and make an effort to stop the behaviour. he’s perfect other than this really annoying trait. she doesn’t correct him either, so it’s down to me to step in.