Just a suggestion (please wait for others to back me up on this) Maybe you could buy one of those 3 day feeder block things, to tryt and ger a few nutrients in the water. I know those things aren't worth the money, but what could it hurt to try?
I've never heard of liquid food. If you find some you could try it though. make sure you keep up on his water changes, if he isn't eating the food it could pollute your water, especially since there is less water in there to pollute than before. If there is a heater in the tank, turn it up a little. Also, maybe you could try hand feeding him with a turkey baster.