Sounds to me like he doesnt wanna stop playing! LOL Bear does that soemtimes to, but usually its a way for him to try and initiate play with one of the humans in the house. Him not closing his mouth shows its not in any way an agressive behavior more of a "touch" to get your attention. Pack dogs will touch one another to interact and communicate with one another. Maybe you could try playing with him outside, then giving him a few minutes to cool off, and go potty once more before going inside, that should calm him down enough so he doesnt feel like all of a sudden the playtime just stops and hes wondering what happened and why he has to go in.