Well I would say your problem are the temps. The basking spot should be from 110 to 120. That would explaine him laying under it and acting sluggish. Get the temps up and let us know how it goes.
If you have a hard time getting the temp up you could place somthing under the light to rasie the tegu higher to the light to get the right temp just make sure its not too hot and they cant reatch the light.
My tegu didnt move much when i 1st got her but now she moves around and makes all kinds of noise she likes to wake up get some heat them starts jumping around till i wake up and let her play in the window. Lol then goes and takes a nap in her home heats up then she wants to eat and digest then its sleep time. Unless she feels lik tearing her whole home up for fun so i can refix it.
Good Luck i hope thing are going better!