We had this same problem, totally random and annoying lol. Our trainer came out and observed and now things are much much better.
First of all don't play with him while sitting on the ground. It is most likely just over stimulation. You need to try to pin point when he is getting to the breaking point and try to stop play before that happens, pet him, calm him down and then resume playing once you are back to a place where he has self control again.
When he does mouth you, say "ouch" with your deep voice, fold your arms and turn your back to him (don't walk away). He might still jump a few times but 9 times out of 10 he will eventually offer a sit. When he does this, praise, praise, praise!! Have a few treats in your pocket for this instance.
You want to keep play time structured for now. Also if you see him getting to his "losing it" point start with some training before he can get there. Work on some basic commands to gain his focus back.
These things have worked SO well with Koda and she was quite the mouther.
If you want to play rough with him, add a cue word. My husband likes to play rough with her sometimes, I do not. So he says "Koda, wrestle" and she knows that means she gets to play with him in a different manner but she never does it to me anymore. And if she tries, I fold my arms and she quits.
I felt like it was never going to end and now it has!
Good luck!