There are several types of lops, and you can usually tell them apart by size and ear length. I'll include a few pictures so you can tell what kind you have.Holland Lop
Weight: 3-4 lbsFrench Lop
Weight: 10-11 lbsMini Lop
Weight: 4 1/2 to 6 1/2 lbs
The size of the ears should help you determine which type of lop you have.
When you determine what type your rabbit is, see what the best weight for him is. Then base his diet on that weight.
Your rabbit should have between 1/4 and 1/2 cup of pellets per day per 6 pounds of body weight. So if your rabbit should weigh 5 lbs, and you think he's a few lbs over that, give him just 1/4 cup of pellets per day.
That's good he gets plenty of timothy hay, and filtered water is good for him.
He should get about a handful of fresh veggies once a day. The best ones for him are romaine lettuce, leaffy green and leafy red lettuce, endive, bok choy, and raddichio. Celery is fine, and so is a little bit of each of the other vegetables listed that I'm going to give you a link to. The types I mentioned above are best for larger amounts, and other veggies should be in smaller amounts. New studies have shown that rabbits should have vegetables with low amounts of calcium in them to help avoid bladdar sludge and bladder stones. Because of that, avoid spinach, kale, and other veggies high in calcium.
I would avoid the pineapple and keep fruit treats to a minimum. Even carrots are high in sugar, so you should just give your rabbit a small piece of carrot or baby carrot one in a while.
While your rabbit is on a diet, giving him a minium amount of pellets and just a small amount of veggies each day is the best way to help him lose weight. He can have as much timothy hay as he wants, though.
A lot of rabbits like to sit in their litter pans. Since he's got the problem with getting messy right now, you can always give him a little bed if we wants something soft to sit on. Some old towels would work well. He'll probably chew holes in them, but it won't hurt him.
One of the ways to help him get through his diet is to give him some cardboard and wooden toys to chew on. I stuff hay in toilet paper tubes just to make it more fun for them to get it out, or stuff hay in a small box and they have fun throwing it around to get it out. I also get inexpensive pieces of wood for them to chew on. If you dip the wood in juice and then let it dry, your rabbit will enjoy the different flavor when he chews.
These are links to web sites that discuss the basic rabbit diet, and give lists of veggies and fruits and treat foods:
Let me know if you have other questions.