rescued lop losing weight

Question
I work for a local humane society and a few weeks ago i brought home a young adult lop that an owner didn't want anymore. There were no signs of illness and he is a very content, calm bunny. I noticed right away he didn't seem to drink as much as my other rabbits, but didn't think much of it. He is a rather large rabbit, but in the past few days I have noticed you can feel his shoulder bones and spine more than you should. He does get a lot more exercise in my home than he did with his previous owner. We let him run around the house for at least 4-5 hours a day. (he is litterbox trained) But I am wondering why he is suddenly losing this weight and what I can do for him. Otherwise he is in good health, and is fed about 1 cup of pellets a day supplemented by lots of fresh veggies. I would really be thankful for any advice you may have.

Answer
All of my rabbits are on continuous feeders that I keep full at all times.  I have found that this type of feeding is better all around.  I feed Manna Pro diet to all of them and I have never had an under weight bunny.  If you are feeding pellets from a department store or pet food store that come in a small plastic bag you are wasting your money.  They are filled with fillers and not very nutritious.  I also feed an unlimited supply of Timothy/Clover hay, which is good for their digestive system.  Check out www.rabbit.org for a super list of good for your bunny fruits and veggies.  Some rabbits will eat/drink more/less than others with no abnormal issues.