I recommend trying to find a local water source that already has turtles in it and re-releasing him back into the wild.
For information on caring for him, read over the main site. It has all the information you need.
http://www.redearslider.com/Basically though, you need to raise the water to at least 8'' this will give him more room to swim around and it will prevent him from drowning if he gets flipped over accidentally. You should try and provide at least 10 gallons of water per inch of shell though, so right now you're looking at 60 gallons minimum. The cheapest way to provide this much space is to purchase a stock tank from a local tractor/farm supply store. They aren't as pretty as aquariums but they provide the room you need without charging an arm and a leg.
Remove any rocks from the tank that are smaller than the RES's head. He can and will eat them if he can fit them into his mouth, this can result in impaction which can be fatal. To provide the most swimming space, look at getting a floating basking area. This will allow you to remove the rocks instead of continually having to stack them up.
Make sure and read through the lighting section on the link I posted. You need to set up a heat lamp and a UVB lamp over the basking area. To heat the basking area, use just a regular incandescent house bulb. UVB bulbs need to be purchased from pet stores. Read the label to ensure it has the proper UV rays and to ensure you are placing it far enough away from the basking area.
Get some turtle pellets. Reptomin is a very good, popular, and readily available brand. Also look through the nutrition section of the link to find which veggies you can offer him everyday.
Make sure and get a filter that is either rated for three times the SIZE of the tank or one that filters 5X the amount of water you have (ie get a filter rated at 300gph for 60 gallons of water). The best/cheapest way to accomplish this is by purchasing a canister filter.
Then make sure you have a submersible heater for the water and thermometers for the water an the basking area. I don't recommend using glass for either of these as the turtle can break them.