Once the tegu gets acclimated it'd probably be a good idea to get in a soak once or twice a week. Just clean out your tub and put in some warm water. Leave him in for 15 or 20 minutes. I know you're doing what you can for in-cage humidity, but this is a good hedge in case the cage just doesn't hold moisture well. It will also be additional handling which will help a little with getting him used to you. Good luck and preserve those remaining digits!