Honestly I would like to say fine shavings because I have had an awful couple experiences inside the enclosure where my ball python took out the mouse and really big chunks of substrate, too. So Since then I have completely switched to paper towels, but in your case particularly, I would say go with the finer of options. That way if he consumes some he can pass it and he won't suffocate either. At least IMO.
Okay, looked more into it: If you can get some pretty good organic cypress mulch fine shavings, I would definitely go with that. If not organic, just some good looking- not pesticide infused mulch. A lot of companies put it in there anymore and won't display it obviouslyNot saying you aren't a careful consumer at all. Just that it's kind of crappy on the companies side.