Your rabbit's ears only flop at the tips, but vitamin A deficiency can cause floppy ears. This site shows a variety of rabbit diseases with graphic necropsy photos, but if you scroll down almost to the bottom of the page, there is a pic of a rabbit in the beginning stages of Vit. A deficiency. My scroll bar shows it at about an inch from the bottom of the page:
http://www.scribd.com/doc/72228408/Rabbit-Diseases
MaggieJ made it a sticky in Rabbit care, so you can also find the site there if you don't want to use the link.
Someone else posted recently about another young rabbit with floppy ear tips, and someone said that it often happens during growth spurts.
You might try supplementing with foods high in Vit. A and beta carotene. (In humans, the body converts beta carotene to Vitamin A.) Foods high in Vitamin A and beta carotene are yellow-orange vegetables and fruits like carrots, pumpkins, mangoes, apricots, summer squash, butternut squash, cantaloupe, peaches, nectarines, sweet potatoes and yams. Turnips, beet greens, romaine lettuce, collard greens, turnip greens, leaf lettuce, spinach, red peppers, and hot chili peppers also have A and beta carotene.