Tough one. How old is this dog?
Some dogs go thru this horrible phase and for some it lasts a good while. It may be because he is in such a phase, or
may be because he was taken from the litter too early and never learned this. (mama and pups "teach" how hard is acceptable to bite.)
Google "landshark dog" or even "landshark GSD." German shepherd pups are notorious for this behavior.
You say he ignores the ouch, the yelps.. and that makes it worse, so I suppose I'd ditch that. Especially if it elicits a growl. It may only be a play growl, but it isn't working. Ignoring is the next thing, but doesn't sound like that works, either. Even a stern "no" isn't working.
I would say always have a toy or chewie to put in his mouth when he begins this and deposit it into his mouth and praise.
If he spits it out, rinse and repeat.
I like my dogs to have a nice soft bite, so I allow them all to mouthe me, but I have found the ouch, yelp to always work except with one dog. Exception dog just would not stop. Those puppy teeth are so sharp. I hestitate to say this, and I am not saying I recommend it to you, and I will get flamed hard core for this... but with that dog, I stuck my finger down his throat far enough to elicit a small gag. He wasn't as fond of making my hands tenderloin afterwards. This wasn't done in anger on my part at all, and was followed up with toys/chews and praise when he took them.