I personally think he'll be fine on mice and rats. There have been studies done on stomach contents of wild tegus showing they eat more foliage as they reach maturity and at three feet I would consider yours an adult. I really think whole prey items are the healthiest way to go. Listen to your tegu if he likes mice and rats feed him mice and rats. I see people here feeding their tegus ground meat made for humans and it's not the same as Bert L. Grinding up whole chickens bones and all. Also you might include a substrate that holds a burrow.