Nems can be tricky but they can also look like they are doing terrible when they are just doing a water change and getting use to their new parameters and environment. I’d give it a couple days before you really start stressing. Just keep a close eye on him. He’ll find somewhere he likes. It’ll move around for a bit, that’s normal. When they deflate like that they are in a sense purging the old water and taking new in. Things to watch out for; constantly open mouth, constantly deflated, letting go completely of their foot hold, and staying in a dark place for an extended period of time. You don’t want to move it if you don’t have to but they can sometimes get themselves stuck in a crevice.
if/when it does get comfortable it will probably begin to roam around looking for a spot it likes. Put your powerhead covers on or keep an eye on the nem. Hopefully all ends well
Nems can be very melodramatic. They won't die overnight. If a Nem is fussing, that's a sign you should check your basics.... temperature, salinity, ammonia, O2, flow. A clownfish can sometimes annoy the bejesus out of a Nem. Don't panic.