I know that you can't feed bearded dragons lettuce, but you can feed them collard greens. So for the past month, I've been feeding him both. I heard that lettuce has a lot of water in them so that's good for him but they have no nutrition. So, collard greens have a lot of nutrition, so I feed him both. (btw, I feed him hearts of romaine lettuce).
I wouldnt feed him romaine or lettuce of any kind. Collard greens are good tho!! Here is a list of thing you can feed and how often, might help ya a bit. http://www.beautifuldragons.com/Nutrition.html