Until you get him into the correct setup try keeping the room temp up to keep the water from getting too cool. Just remember that the sooner you can get him into the proper habitat the better off he will be and the less chance he will have of getting sick.
Start by reading here:
http://www.redearslider.com/index_about.html
When getting a tank/habitat set up for him, try going as large as you can now to save you money in the long run. Remembering that when fully grown he/she will need between 90-120 gallons depending on sex will help. The minimum size tank he needs now is 30 gallons but you will have to upgrade as soon as grows, so go larger if at all possible.
Insuring that he has the correct water and basking temps are extremely important in his habitat to keep him from becoming ill (which is why getting him into his permanent home ASAP is important). For his size, the water temp should be between 72-75F and the basking temp should be between 88-90F.
You will need a basking area that will not take up too much of his swimming space (floating dock or something similar) with the heat light and the UVB light directly above the basking area. Many on here use a regular household bulb for the heat light and a UVB bulb that emits at least 7-10% UVB rays. As mentioned above MVB is an option that takes care of both but due to the high heat it produces it shouldn't be used in tanks smaller than 50 gallons.
You will need a filter rated for at least 3x's the size of tank you have. Filters are rated for fish which are not as messy as turtles. Investing in a good filter is a must.
A heater is a must since a turtles health depends on his water temp not becoming too cool.
You will need to buy a good reputable pellet as the staple of his diet. Pellets should be fed once-a-day every other day for a turtle his age/size and only the amount that would fit inside his head if it were hollow. Veggies should be fed daily especially dark leafy greens such as green or red leaf lettuces, romaine, dandelion leaves. See the feeding sticky for more info.
Hope this helps. Read through the forum and main site as well as look at some of the great pics in the Photo Gallery section. If you have any more questions just ask.
