Sounds like he is constipated and or poop stuck, try putting him in warm bathwater in the sink and wash his little butt. And try a little canned pumpkin. Rosi had that when she was like 10weeks old and nobody told me to keep her butt shaved, she was salvating and crying, ended up taking her to the ER middle of the night where they gave her an enema...well 160.00 later and a very raw little butt she was fine, but it shure scared me. Trust me after that i made shure she got Buttcuts
Hope thats all that is ! Good luck and keep us posted !