I mean, I'd just suggest Robert's stuff, like Level 365, Sightless Seers, Level 710, and so on. I might have a BIT of a bias there though xD
But aside from that, I think there's a "hodgepodge" of levels he could travel to. Level 47 sounds like an incredible one. Level 154 would add a lot of suspense if done right. Level 666 could be used as a way to explore his deepest fears and give some interesting character development. Level 906 could be a nice little break from all the horror and danger (and I know a lot of people like Blanche) As for how to get to these levels….It's the Backrooms, if there's not a way, then you could always add some form of reality-warping entity or object to explain that, which could also add an interesting element to the story.
As for entities, you could really spice it up. A lot of the really "iconic" and common ones are found most often in the lower levels, so just pick a few of those I guess. Phobic centipedes would be REALLY good for horror if described in the right ways. Dreamweavers and entities like them could show that even dreams are not safe in the Backrooms. The Keymaster and the Red Knight are some interesting ones as well.
As for groups, M.E.G. is the big one, though your character could get tangled in a conflict involving two groups. Might add some more interesting conflict, rather than just the typical horror stuff.
There are numerous objects you can use as well. Almond Water (of course), firesalt, reality fresheners, liquid silence. There are way too many good ones to list.
Regardless of all this, I'd think about what would make a good story plot. You can pick one or two things from the Backrooms to make the main plot, then add in other things in throughout, even if just in passing mention, since there's so much.