It's possible to get a used tank (cheaper) through classifieds, thrift shops (the Salvation Army in the town next to mine had a nice 55 gallon one with a screen for $50.00), garage sales. Try also
www.craigslist.org or
www.freecycle.org You may not find one, or it may a while, but it wouldn't hurt to be on the lookout...
My RES is almost 6.5 inches long and I keep him in a 110-gallon stock tank that I got for around $45.00. It's wonderfully roomy, easy to clean and strong. A few months ago I found a 100-gallon stock tank in the classifieds for $10.00. You might consider one of these as well. (For either a regular tank or a stock tank, you're ideally looking at at least a 90-gallon one.)
I'd try to up the basking area a few degrees, to 88-90F to see if that doesn't encourage him to bask more.
You said his shell has gotten worse since you've had him. It could be that it's looking worse because you have given him a much better habitat and now he is growing, where he wasn't before. Sometimes these things look worse before they get better...
Even though this link is directed at tortoise shells, read toward the bottom and you'll find some info about aquatic turtles and their shells.
http://www.turtlestuff.com/shellcare.html Perhaps giving him an occasional "bath" would help him slough off any old scutes or pieces.
I can't see your pics from the computer I'm on at the moment, but will look at them when I can get to one where I can. Thanks for posting them.
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-