How to stop play bite ?

Mine is a medium-breed 1 year old boy, he always play to play bite the smaller dogs. Normally he will chase them, and they run, when he catch them, he will play bite their ears and necks.

I always go beside him pull him away because other dogs owners sure dont like and will push my dog away. I also say "NO mouthing" to my dog, but after 1-2 months, he still do play bite.

At home, he always bite me.

Any advise?

At his age he shouldn't be biting you !! Most dogs play mouthy-mouthy which isn't biting but playing. More info please ?

Ah, why is your dog roaming around with other dogs that aren't your own? My dogs play fight ALL the time literally. Only you can decide if he's being too agressive, it sounds like maybe he's just playing. I can't really help you with the biting other dogs isssue, but when he bites you and you don't like it, ignore it. He's trying for attention negative or positive, by reprimanding him he's getting attention, exactly what he wanted. On the other hand, if he's biting hard and hurting you and quick snap on the nose should do the trick.(some of my fellow dog owners also like the spray bottle)

Mason play bites my arm. It doesn't hurt at all; he knows not to do it with any pressure. My girlfriend doesn't like this. Is it feasible for him to understand with me, it's okay, with her it's not?

I would say yes, there are some trainers I know who would say no, because it confuses the dog. My girl licks my boyfriend incessantly but knows better than to do so with me.
I would say yes. You can make it clear that it's ok for her to play bite with you and not your girlfriend. But, make sure your girlfriend is the one who makes this point clear to your dog, if you stand up for her Mason might see her as the vulnerable one in the pack. Just tell your girlfriend to grab a toy and let him bite that when he tries to play bite her.