Uni does that too. that's what I think is she wants to keep playing. I think he's just a pup that found his teeth
You can tell when they are really PO'd they show their gums, and do a continuous growl, even the eyes look mean. But when they are just playing, it seems like a purr/growl, and the biting is just a quick nip, then a bounce back. Also when they're playing you can see the body movement is bouncy, jumpy, whereas when they are mad or scared the body is very stiff, ears down.