Just be careful to avoid getting any liquid directly in the snakes glottis (breathing tube). It is a small tube near the front of its mouth that can be extended externally when swallowing food to allow the snake to continue breathing. Liquid that gets directly in the glottis travels to the lungs where it can easily cause RI.

Snakes have swallowed worse things, I'm sure. If he goes for an extended period of time without a poop, I'd be concerned. It may signify an obstruction. Just keep an eye on that. If you get concerned, take him to the vet just to be sure everything is working okay.

If the snake is young, or if you are afraid of this happening again, perhaps you would consider switching him to a separate feed-tub for meals? I've had a lot of success with it, personally, and no worries about eating substrate.

I hope something I've said will help.