hopefully, he will come out of hiding.
if all you did was add 2 cups of aquarium water to the bag, that is not good enough for acclimation purposes. your salinity in your tank should match the salinity in the bag water as closely as possible before a fish is released into your tank.
it's possible the bag salinity was much lower than that of your tank (many stores keep lower salinity). going from low to high drastically can shock/kill a fish.
on another note, did you dump the whole bag (fish and water) into your tank?