You are feeding him too much they can go every other day. Also Oscars are lazy don't expect to much movement. You are GOING to have to move him in a bigger tank. It sounds like he is stressing. Check your amonia also. Uneaten food creates amionia which is pretty much the number 1 fish killer. If it is hugh you will need to gravel siphon. Also you're nitrates are prolly gonna be high too- Do a 1/4 water change. Basically TEST you're water! Also, get him in a bigger tank ASAP! He will grow faster then you know.