I would go with Chicken, just because I, personally, don't like to feed too much fish.
Whichever is the least expensive per pound, which is probably the 18 pound bag, IF you are sure he'll like it. But check their return policy...I think you can probably it for your money back just in case, as long as it's half full and you do it within 30 days.
Also, you mentioned he is out of dry food until Friday., AND that he always seems hungry. What are you going to feed him until Friday?? And is it possible that you haven't been feeding him enough and that's why he has lost weight? If he always seems hungry, I would guess it's because he isn't eating enough, particularly since he isn't even maintaining. Some cats, especially if they are really active, need up to 35 calories per lb of body weight (what they SHOULD weigh, not what they actually weigh) One bowl of food doesn't really tell us anything, because it depends on what the food is, the size of the bowl, and any other food he may get
Maybe this article will help you in determining how much he should be eating: http://www.thecatsite.com/a/how-much-food-should-i-feed-my-cat
and generally rule of thumb is about 20calories per pound of body weight. if active, more is best. id talk to your veterinarian to figure out his weight and calories
