Its the iceburg you want to stay away from. Other lettuces are fine. Like, romaine, green and red leaf, butter, frisee, and many of the 'artisian' lettuces. The darker the green, the more vitamins it has in it.
Kale, spinach and the other high calcium greens can be fed like 3 times a week to adult rabbits. Since Charlie is still young and not fully grown(under 6 months) he can handle the calcium better than an adult. Just because you feed the buns kale and high calcium things more than 'recommended' doesn't mean that they'll immediately get sludge. Bladder sludge doesn't always happen because of food, sometimes it happens to breeds that are predisposed to it. Sometimes it just depends on the bunny.
You want to watch out for things that are high in oxalic acid. They happen to go hand in hand with the things that are high in calcium too.
I limited the high calcium/OA stuff to every other day and mixed into the rest of the salad so it wasn't just that one thing.
The list that I've compiled, the things that are high in oxalic acid are:
parsley, spinach, mustard greens, beet greens, swiss chard, kale, raddish tops, sprouts(1-6 days after sprouting, that have a high level)