I would try to limit how much he ingests. With certain substrates frogs (or any animals) can develop gut impactions that occur when they swallow too much bedding and it becomes impacted in their intestinal tract. I have never heard of this happening with spaghnum, but I would actively try to not have him eat bedding. As for the other substrates, my favorite bedding for Ceratophrys is moist paper towel. It's not very attractive, but they can't eat it, and it can be changed easily. With large terrestrial frogs like this you need to change the bedding and water very very often so ammonia and other nasty things don't build up and kill your pet. That being said, spaghnum is my second choice for these frogs. Maybe try feeding in an area with no bedding? Either way, a little spaghnum will not kill him if ingested.

Alex