try some blood worms. I had a couple of river cooters that i couldn't get to eat and i gave them a cube of bloodworms and now they're eating everything in site. Also, throw a couple of plants in the tank with them. Unfortunately i dont remember the name of the plants that i put in with my kids but they eat them up. There should be a post down there somewhere regarding my problem with my cooters, and everyone was really helpeful. hopefully that will help you.
also, to the best of my knowledge, turtles tend to float when they dont eat enough. At least thats what i was told. It seems to hold true. Also, when he floats is he floating sideways? i had a hatchling RES that in 2 days lost his appetite and was floating sideways. he got a batryl shot every other day for 5 days and it worked wonders. good luck with the little guy!