should I just use the "stop playing and ignore him" method to show him it's not okay to growl?
I just re-read your post and this caught my eye especially. Ignoring would be okay, but in general..... if your pup ever growls and it's not in play, (I hope you can distinguish by his body language and facial expression. You should study up on that...lots of stuff online) it is bad idea to ever show him that it's not okay to growl, as with some kind of punishment. Some people scold or slap or yank or scruff to tell their dogs that it's not okay to growl. On the contrary: It
IS okay to growl. That is how dogs communicate that something is bothering them, something is wrong. And we should listen.
My son's dog growled and snapped at the air when I clipped one of her nails which I didn't have a really good view of...not good enough. Big mistake. I cut her quick badly on a nail that was already split. It had to hurt her big time. If she had been punished for growling in the past, she may well have realized that growling doesn't work to make people back off and she may have taken my face off. I thanked her for telling me, said I was so sorry. And let it heal before trying again, at which time I had to re-condition her because she did develop a fear of that back foot being messed with. Now she's all fine again.
So, if you ever have a dog growl at you or anyone else, don't punish him. Find out what is going on and fix the problem. For example, some dogs growl when people try to take away their toy or food bowl. If they get punished, the next time, they won't warn and may well go right for your face with those big, canine teeth. So, instead of focusing on that growl, you need to condition the dog to enjoying having people around his food bowl or taking his toy.(preferable right away, while he's still a puppy, very important) There are ways to accomplish this (and most other normal dog behaviors that we humans don't like) which teach the dog our ways without disrespecting their ways and without causing more defensiveness or whatever problem you're having.