Update...
Contacted the vet yesterday, and to my surprise, they said it was "not uncommon". (Funny how I've never seen anyone post about it before, and have always heard how safe it is to feed on paper towels or newspaper.)
The vet said to give 5 or 6 drops of mineral oil daily for 3 days, and it should break down and pass.
What he didn't mention, (and I didn't think to ask) was the possibility of rectal prolapse. I've seen this in several young boas in pet stores. Other than the mineral oil, I guess there's nothing I can do but hope he expels ONLY the stuff that should be expelled.
So, he's had his first dose of mineral oil, (and he enjoyed that about as much as I would have, I think!) so we're back to "sit and wait".