I was in a similar situation with my Black Tortoise. The store that I bought him from was only feeding him lettuce and endive. I talked to a few people and decided to try the following, which worked perfectly for my little tortoise, and wild caught Chuckwallas (Which are the most selective eaters I've ever seen).
Take his regular food and chop it up very fine, as well as what you want him to eat. I use a mixture of collard greens, forest and grassland tortoise food, dandelions, and prickly pears. Mix all of this up and serve a bit damp.
It may take a while to wean him over to what you want him to eat, but it will happen. The key, in my opinion is to stay firm with what you offer. And also reduce the bad stuff each feeding.
I was very surprised that it took only a few weeks to completely eliminate the bad stuff from my little torts diet... Also his favorites.
The grassland tortoise food that I use has a good amount of Timothy Hay in it, along with other nice healthy ingredients. When wet / damp it puts off quite a strong aroma. It is also finely chopped and sticks to other food quite well.
You might also try blending his normal diet with some hay in a blender. Of course you would want to squeeze or strain the excess water out. But that might be another approach.
Good luck,
-Kory