Since you just got him & it's too soon to handle him, I'd do this during the day (that's when HE sleeps) & when he's hopefully in a hide- I'd just put something in front of the doorway to the hide, so he's contained & not frightened by seeing what you're up to.*
Then just change out some of the substrate & mix in the dry as best you can- don't worry about "perfection"-you can pick up the water dish or other things to mix the substrate by hand, but don't take all day- try to be fast & the least disruption possible. Put his furnishings back as they were & remove his temporary "door" on his hide- okay?
He'll probably poke his head out at some point afterwards, but he shouldn't be very bothered by this. Any more than if you just did a "spot clean" to pick up feces he might leave. Good question, btw.
*For future reference- you can also do this (cover the door of the hide a snake is in) when you have to change the water bowl, & don't want a hungry snake coming after your motion thinking & hoping you might be "edible", lol. Cardboard is a bit too lightweight (easily moved by the snake), but many things can be used- I have large & "enthusiastic"- not "shy"!- rat snakesfor which I use a ping-pong paddle to block their doorways- it has a nice weight to it-
& they respect it- never push it out of their way. But eventually they would- it gives me just enough time to tidy up or clean their water bowl before they start wondering what they're missing, lol.
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