tekniq, you've had this little turtle for two or three weeks, isn't that right? And he was eating at first and was active, but now his behavior has changed?
If he's lethargic (acting tired) and not eating, then it really sounds like he's ill. There are lots of possibilities, of course. turtles are like us - different things can go wrong. One possibility, since you haven't had him long and he came from Chinatown, is that he's got a respiratory infection. It's fairly common with little turtles from Chinatown. But whatever the cause, he'll need to see a vet to get a diagnosis and treatment. This isn't something you can treat yourself.
It's most worrisome that he's not eating. He needs to get some calories in him. Here's a link on enticing a turtle to eat. Since he's ill it would be ok - just temporarily - to feed him things that turtles really love but that are usually just reserved for treats since they are pretty rich.
http://www.redearslider.com/entice_eating.html
Keep him warm, clean and avoid stress. You can bump the water temp up to 82 or so.
Good luck. It's not anything you did wrong - little turtles are rather delicate to begin with and the ones from Chinatown seem to have a bad track record where health is concerned. We'll be thinking of you and Constantine and pulling for you. Keep us posted.