You can switch him.
With patience I've been able to switch lots of reptiles over to the food I wanted.
Take the food he likes, mix it with the food he should be eating, and chop it up real fine. And I don't mean 1" squares. I mean finely chopped into 1/4" or smaller cubes. If the food he likes is pellets, soak the pellets until they are mushy and then add in the finely chopped new stuff and mash it all together. In fact, let the chopped up meal sit for an hour to let the juices and smells mix together. Then feed it to him.
At first, make the new food only 10% of his meal, then 20%, etc.
Don't try to do this all in one week. Give it a few weeks. Unless you are currently feeding him hot dogs and nachos, he'll be OK on the "store bought" stuff until you get him on a more varied diet.