Help getting my dog to stop chewing on me?

So, he's not my dog, he's my grandmother's, but we see each other often. I've really bonded with this dog over the past year and a half-ish that she's had him, which is something I'm proud of because he was incredibly afraid of people when he was found (he'd been dumped on a country road). But now that he completely trusts me, he always chews on my hand. And I think it's in a sweet way, but he's a big boy with big teeth and sometimes it's too hard and he doesn't understand it can hurt. I've tried redirecting his attention to the many bones and chew toys he has available, and sometimes that works, but he usually ignores the toy I'm giving him and goes back for the hand. He does this ALL THE TIME, and I hate getting onto him, so I just kind of let him do it until it hurts and then I'll yank my hand away with a dramatic "OW!" and leave, to try to show him that too hard means no more. But he doesn't get it. He'll follow me and try to take my hand again. And he'll only really stop when someone with a more authoritative voice tells him too. He just seems to see me as a playmate rather than someone in charge.