teeth grinding when a rat is happy, relaxed and content is bruxxing. There IS stress bruxxing but its much louder and the rat's body language is tense.
Wait until you see a boggle, you'll love it!
When they bounce around like that its called popcorning. Depending on your boys age, I would NOT poke, and scritch their bum. This is an aggressive move on another male's part and if your boy is going into the hormonal phase it could set off a bite or at least huffing/puffing and rubbing.
If your boy is young he just needs to learn teeth are not acceptable during play..."Eeeep" when he nips you or uses his teeth too hard...this is rat language for "stop, that hurts, I don't like that!"...they often stop and look at you, and it may take many times but often works eventually
With girls you can poke rumps and they just bounce more
For playing I find the safest and easiest way to initiate play with both sexes is put your hand (from the front) over their head and shoulders and give a gentle shake, use a happy voice and say something like "wheee" or whatever that they will associate with playing eventually. they often will bounce forward and thats a great sign! Once they have done that they will shake their head, popcorn, run away and then come back for more
Here are 2 playing videos I have...the first I did up to show a girl how I play with my baby girl rescues. They didn't cooperate enough to show the hand over head much, but you'll get the idea.
The other was my beloved orphan Dilbert, who was the best handwrestler in the world and started when he was 3 weeks old!
You MAY laugh at my commentary, everyone else does LOL