I would think that this posture is a sign of he wanting to play with you. Is his tail wagging a thousand miles per minute? What is his expression like? Excited? Whining? or Still? Is his eyes giving you a deathly stare? Does it have tears in it? Or is he giving you puppy dog eyes?
Maybe his tummy hurts?? I usually do that when my tummy hurts, but that's just me.
