All of mine have always eaten rats. The one GTP I ended up keeping is a pretty petite girl and eats small rats, but the next time I order I'm going to bump her prey size up a bit. I remember someone on the forums years ago that fed a hamster once per month. As for being fragile...mine have always been pretty hardy. I had a subadult WC biak I got at a show years ago that ate about an hour after her 5 hour car ride home. She was pretty gnarled...big scar on her side, had to help her get rid of some parasites, but she never missed a meal and was very easy to handle (which was good for me cause she was huge). I'm far more worried about getting my new bp to eat than any GTP I've ever had. I think out of 9 GTPs only 1 gave me problems feeding, and he was also the only one to ever bite me.
Did your little guy start to change yet?