What else is in the over all diet -- try to list everything that has been eaten in the last week?
Try mixing one part red lettuce and one part collard together. Cut them in to small pieces then mix them well before putting in front of the ig. This way there is no way around eating good stuff. Over two weeks or so work out the red lettuce and mix in other foods.
As long as the right food is there it will be eaten, eventually. Don't let yourself be trained too much and only feed lettuce, bread, and fruit. If you have a good Ig-Den or MK salad in front of him and he does not touch much of it for two or three days no long term harm will be done. But you still have to refresh the salad daily, in an ig cage climate the produce rots within hours.