Make a slow switch by adding larger and larger amounts of the new stuff over time.
For example, week 1 make the meal 95% the old stuff, and 5% the new stuff. And really chop the new stuff up real small and mix it in thoroughly.
Week 2: 90%/10%
Etc.
If at any point he balks, then slow down. There's no need to do it quickly - he won't die from eating greens.
And don't switch him completely to something new - keep it a good mix of grasses, some greens, and a little tiny bit of the commercial stuff. Consider other stuff like weeds (dandelion, clover, etc.), cactus pad, maybe some soaked timothy hay.
Good luck.