I not think the consistency of BM's good enough reason to deny request to train. I do think if he's not ready, that relevant. Some people have hard BM's, some soft, and normal changes per person. If your issue is cleaning the potty after his BM's, then train him directly on toilet but it seems wrong to deny training if this is the real issue.

I've said to parent that a child not ready and I have document detailing what I define as ready - child need be able to pull pants and underwear up and down independently, child need be able to follow instruction to go to toilet, child need be able communicate if need go. I've never denied based on consistency of BM being loose and less desireable.