Actually, to me, it sounds like something is hurting him. Maybe he's slightly constipated and it's just hurting him to poop. Maybe something is hurting him to curve his back when he poops. I noticed the past few days Nova has been trying to poop without curving her back so much because she got spayed and I guess it hurts for her to be in that position. If he continues, I'd take him to the vet.