For best results, I would FIRST offer him a live pink at the first feeding (ALWAYS better to feed what is familiar food) but you could have a f/t thawed one ready
to feed at the same time...if he takes the live one, wait until it's swallowed & offer the f/t one from tongs w/ a slight wiggle. He'll most likely take it.

Next feeding: you could try just offering the f/t, IF he took one at the first feeding after the live. If not, repeat above until you can switch him completely. Patience.

Aspen will probably work OK. For what it's worth, I always use Carefresh for rosy boas, they like to burrow in it and it's hygroscopic. Carefresh now is compressed,
(years back it wasn't) so you manually have to work it some by hand, & I add in some paper shreds (my clean papers shredded) to lighten it up some (& recycle).
When I've raised rosy boas on Carefresh, they actually burrowed tunnels into it (like an "ant farm" ) because it holds together enough...they like that, they feel
very secure, though like I said, aspen will probably work ok.

As far as cage furnishings, they aren't like rat snakes about climbing, but they will appreciate lower things (like driftwood**) to climb over & hide near, & tunnels*.
*While they're small, you can even use the cardboard rolls from paper towels, & partly bury them in substrate. **Just make sure nothing has a small hole in that
they can just barely fit thru...as they grow (or even with a meal) they can get stuck & because of directional scales, may not be able to back out. I recommend
plugging such holes (if any) no matter what kind of snakes you have. (you can glue a small cork in the hole...the color comes close to driftwood)