Hi My Boyfriend Was Shaking Off His Shorts This Morning

hi my boyfriend was shaking off his shorts this morning when i noticed a big spot of blood on then "backside "
now we have gone to many of drs because he has a constant pain on his right side sometime sever pain he is in tears but dr say nothings wrong . well first they said kidney stones but that was a long time ago .
hat can it be , i know yesterday we move so he was doing heavy lifting but this is all the time he stops but this time it was bad