When I did my first feeding on loose substrate and the snake swallowed a little, I freaked way out lol.
Truth is, unless they go around basically shovelling all the substrate of the cage into their mouth, a couple pieces here and there won't harm them at all. If they can digest bones and sinew and cartilage, a little bit of plant fiber is really no big deal for them. Don't worry about him, he'll be fine
I feed all my snakes on loose sub and never had any problems, even though from time to time they might eat a couple pieces of bedding. Once they've done a feeding or two on loose bedding then they get the hang of it and don't tend to get it in their mouths very often. Here's a trick though if feeding on loose substrate: thaw the feeder in a plastic baggie if you use water to thaw them, because that way they stay pretty much dry and less bedding will stick to it.