What I do is fill thier dishes with water and put the dandelions etc. in them like a vase. I've found that they like the shallow faux rock dishes the best. Using this method my dandelions usually last 24 - 48 hours. If he's only eating dandelions, I'd try to give him as many as he can eat (probably 5 , and try to pick the smaller ones) until he gets his strength up. Nasturtiums are also a big favorite of my chucks. They actually seem to like the chopped leaves as much if not more than the flowers. Another favorite are the lavender wild raddish flowers. All the water in the bowl helps keep the salad moist and the chucks well hydrated. I agree with the post about trying insects too. When I got my little guy he was underweight, but slowly gaining it back with the veggies, once I added mealworms his rate of weight gain increased dramatically. Also, I try to change out the food bowl daily because the water and warm temperatures seem to accelerate decay.
In terms of broadening his diet, keep offering other things in conjunction with the favorites. Once he's healthy, start cutting back on his favorites to encourage him to branch out. My chucks also seem to like variety and will even lose interst in favorites if they see them every day, so keep trying new things and try to offer a slightly different combo each meal.
Here are some other common garden plants I've had success with:
Yellow pansies
Moss rose
Hibiscus
Cosmos (the lavender ones)
Mint leaves
Geraniums
Cat's ear (Hypochaeris sp.) these are dandelion like flowers, because of their smaller size though, they're really good for little chucks
Mustard flowers
If you read some of the past threads on feeding, you can find a lot more suggestions. You may have to offer way more than your guys can eat to make sure that the iggy leaves enough for the chuck. In terms of supplements, I've had the most success with the T-rex Iguana Dust, but I'd hold off on this until he gets his weight up.
Good luck!
Alice