red underbelly, help?

[Kinana]

He may be healing. If it doesn't look like it's getting worse, then leave it be. Don't pick him up, and replace his substrate with something soft. Snakes underbellies are sensitive.

I know it's debatable, but what's the best substrate to use then?
I was previously using reptibark, but I switched to Aspen shavings.
I know not to use newspaper print or gravel bits.