It sounds like he probably needs more stimulation.
It is possible though that something else has triggered this, such as a change in his circumstances, maybe an illness, maybe an addition to the household, maybe a change of routine.
How big is his cage? Does he have many toys in there? Do you vary what he has?
For the litter tray in particular I would suggest maybe getting one, or adjusting this one, so it attachs to the side if possible, so that he can't flip it (although he might still take great pleasure in digging it).
Stamping can mean anything. Typically it means fear, but for some buns it can be a way to communicate anything from excitement, anticipation, annoyance, and others, to just seeking attention from someone.