He looks great! A friend of mine target trained his tegu with a bell. It was an Argentine, though, so a bit easier to work with, but I think if he can do it then you can too

Tegus are much smarter than people tend to think. I think their slowness and tendency to ignore certain things makes people think they are dumb.
Hope all goes well for you! He's at a good size for that kind of training, from what I understand.