'He" will be a he until there is another clownfish. Then "he" will become a "she" and become quite territorial.
Safest for your tank to quarantine your new fish in a separate, bare tank for a couple of weeks at least. Conditions you describe at your lfs are brutal, and ich is very likely. If your fish is qt'd, he won't transmit ich to your tank and kill everybody else.
Do not offer food more than once or twice a day. Ask your fish store what they were feeding, and offer that. Frozen mysis is good for tempting a reluctant eater. So is a tiny, tiny piece of a raw shrimp from your grocery store. But do not feed your fish more than will fit into his mouth for one big bite.
Also---your nitrate is still higher than it ought to be. Not too bad for only fish, but lethal for corals. If you are running any filter [instead of live rock] or bioballs or sponge or the like, that can cause nitrate to stay high. HTH.