what kind of water conditioner are you using? some are better than others for removing high amounts of chlorine like you said you have. if you aren't using it already, i would recommend Prime. it does more than remove chlorine.
also, if he was in a 1/2 gallon bowl up until a day ago, and you were only changing the water every 2 weeks, that is not near enough, and he could have several problems resulting from that. for a 1/2 gallon he should've had the water changed every day.
it's good that he is now in a nice big 10 gallon! however, if it is not properly cycled yet, the water parameters could be all over the place, which could have killed your other fish, and could make him sick.
i don't know much about cycling, but a lot of people here do. but it's a big process, that if you don't do it right, can really harm your fish.
good luck, and hopefully someone can help you more than i can. :)