There's a list of what to feed your RES under the feeding and nutrition section at the top. Also there is a what not to feed your turtle. Many people feed romaine, green leaf, and red leaf lettuce, shredded carrots, escarole, radicchio, squash and many other yummy veggies. Fruit should only be given once a month. Foods high in protein should be given on a limited basis as well (feeders, crickets, worms, shrimp, krill etc) Feeding too much protein too often will cause fast growth and health problems. It's important for them to have a balanced diet so offer veggies daily even if he doesn't take to them right away.
Placing him in the sink may not be a great idea. If there is any soap residue at all, it can seriously harm him or kill him. Soap residue is extremely hard to rinse of so be very careful. And remember to always disinfect the sink after he's been in there for your safety as well. If you put him in another container such as the sink or kiddie pool, be sure that the temp is the same as the tank(78 degrees for hatchlings,75 for older turtles with the basking area around 90).
Remember to plan ahead when making purchases such as tank/filter/heater etc. Ten gallons of tank per inch of turtle should be the minimum size tank and as he grows his home should also. RES can grow to be 6-9 if an adult male or 10-12 inches if adult female. Buy as large of a tank as you can now to save money on the upgrading costs as he grows.