Aww, how cute!! He looks very comfy, lol.
Harmony and Destiny do something similar. They like to be cradled on their backs like a baby. So anytime you are sitting and you happen to have your arms crossed or in a similar position, they will jump right into your arms and then fall backward so they are lying on their back.
We've had to warn some guests that they do this because they wouldn't expect them to do this and would unfold their arms and my babies will fall. They just actually expect to be held this way.
Destiny in fact loves to be carried in my arms all day like this. She will fall right to sleep with me holding her cradled on her back.
Only 2 dogs I've ever had that liked being on their back like that. Most hate it and won't let you hold them like that.