Mystery snails can be very odd little creatures, in my experience. It is completely normal for them to go dormant from time to time, particularly when adjusting to new parameters, or if they're full and not in need of food. The flipping over part just goes with the territory. . .
That said, these snails are particularly sensitive to changes in nitrate, so it is a possibility (hopefully not!) that if he was not properly acclimated, he could be in trouble. Not much that you can really do for him but watch, wait, and hope for the best. . . I hope he's just being a normal fat sleepy snail!