Are you doing the cooking for him? Can you sneak some veggies into his food? Like a little onion and/or tomato into the omelet?
2-5 years? Pfft. Not worth it.
Yes, thanks to this thread, I suggested he might want onion and peppers in his omelet and he agreed.
That’s a really good start. If you can get him used to that, it’s a victory! Wait until that becomes a habit then try another small step.
Woohoo!