Finally, at the 37-day mark, he pooped and peed on his own - very explosively, I might add - after I had just finished deep-cleaning his tub and was about to put him back into it. So I guess the moral of the story is, continue keeping a close eye on him, but also no need to panic yet. The little ones seem to be able to save things up for a very long time.
So what RedRabbit is saying is the best way to make your snake poo is to deep clean the enclosure
2 BP's, one ratsnake, 2 dogs, 3 cats, 2 small caged birds, 7 chickens, and a toddler in a pear tree