With that much info, it's hard to say much at all. He sounds like possibly a playful boy and maybe he's too preoccupied to play with you when others are around, or you treat him differently & he knows you won't play when people are there?
Re reins, perhaps you're on his mouth more than you realise or having any contact is too much for him & he hasn't learned this well enough to be comfortable with it. Maybe his mouth hurts. Maybe he's uncomfortable or unbalanced, from saddle, you, etc & feels the need to stretch out & hollow his back to try to avoid the discomfort.