I taught my puppies by way of social learning too.
I was very sociable with them if they were exceedingly gentle in their mouthing and if their mouthing came anywhere near close to causing me pain or discomfort, I became instantly anti-social. I removed every bit of social interaction that they loved; looking at them, speaking to them, scritching them behind the ears. In fact, I became
so anti-social, that I completely removed my self from the area for a full 30 seconds or so.
In addition, I would make sure the puppies had a chew toy to use instead of my hand. And when I'd walk away, I'd make sure they had their chewy so they'd learn that there
are things they can gnaw on to their hearts content. After that firm, clear feed back, I'd give them another try. If they were gentle, I gave them straight information that being gentle
worked to keep their person around, along with all the attention, play and even tasty treats.
What I didn't include in my social dynamics was the idea of me instigating
every interaction. I know that's what a lot of people say to do. But I don't believe in the idea of dogs and dominace, dogs being so eager to take over if they're allowed to make any decisions etc...I don't instigate every game or invitation....just like when a friend asks me if I want to go for a hike or shoot some pool, I don't decline because I wasn't the one to instigate it. If I want to play, I'll play. If I don't, I'll say "no thank you." And that goes for human friends or dog friends.
Later on, once my pups would catch onto the need to be gentle in order to make it work....make me play, I'd then insert cues to start and stop the game. I didn't particularly want my dog coming right up to me or someone else and start mouthing or drooling all over them. So, I put it on a cue. Their way of inviting the game had to exclude immediate drool or tongue on me. LOL. They had to ask first, then maybe we'd play. They let me know if they want to play by play bowing, play growling and other funny social antics. If I want to play at the time, I'll get in on the game. If I'm busy, I'll just be friendly, but not join in and they then find something else to do. This is just the way it always is in my house and they've caught onto this social dynamic.
I'll hold Chuli on her back, cradled in my arm like a new born baby. (she likes this) And I'll wiggle my fingers in front of her face and she gets all silly, grabs my fingers in her paws and begins to take them into her mouth, but doesn't bite. She just pretends. It's funny. She immediately spits my finger away. She doesn't really want to bite at all....just pretend.
One thing that is very important in order for your dog to learn is
absolute consistency. And yes, sometimes the "yelp" is construed by the dog as your joining in to play. Since that isn't working, I'd eliminate that and simply and immediately walk away every time and everyone who interacts with the dog must do the same. Come right back after about half a minute and give your dog another try. Repeat as necessary. When he mouths gently, go ahead and give him attention. And I mean EXTREMELY gently. The more tries you give your dog, the higher the odds that he'll hit on the right behavior where he CAN be reinforced. (where you WILL be sociable) So, don't do long, drawn out time-outs. They defeat the purpose of the time-out. What you're doing is demonstrating (socially) that hard, obnoxious mouthing won't work. But gentle will. Once he's getting good at it, if you don't want him instigating it by directly slobbering all over you, put it on a cue and invite him and don't
reinforce by giving attention when he just comes up to you and starts mouthing you.