9 y.o. back hurt on trampoline - serious or temporary?

My nine y.o son fell onto his knees on our trampoline yesterday and said he heard his back crack "like bubble wrap".  he was a little stiff but seemed okay the rest of day.  today he played basketball, skateboarded and again jumped on trampoline - nonstop all day.  After these activities stopped, he began complaining about his lower back, then his neck.  he was very stiff in the grocery store and chose to ride on the cart leaning against me, holding his back.  he is on the couch on an icepack, and took an advil but rates his pain a 9 on a 1-10 scale.  At father's insistence he got up to move around and use the bathroom.  He said it hurt in his lower back "really bad" when he went pee. is this a simple muscular thing taken care of with rest and ice?  or do I need to get hime to the er?