I have no idea what's going on with him but either way, I wouldn't breed him. If he may have a legal gene or a gene that can cause serious problems I wOuld not breed him. I also didn't breed a mouse that's having a serious issue like that for precautionary measure.
Would you say the gross things looked kinda like blood clots? Or bloody stool(a bit diarrhea looking, maybe)? Get a picture maybe?