Hi, apple snails can float for a few reasons. In the wild they close up and float down stream as they look for food or want to move on, they can float if they're being picked on? The water conditions can be off? Air trapped inside the lung have you tried turning him upside down and rotating him under water? This would help the bubble come out.
I have had a few apple snails that have done this before so it's not unusual.
70 gallon elephant nose tank.
30 gallon boraras , red longfin bristlenose, bamboo shrimp, crs,rcs and different snails.
7 gallon halfmoon betta and red ramshorn.
70 gallon elephant nose tank, Gonzo the elephant nose and friends!
30gallon borara merah and brigittae green, red longfin bristlenose, shrimp, ninja, taiwan,bamboo, and snails.