My rabbit appears to keep his weight in a healthy range. He is 8-11 years old. He is not neutered, is arthritic, and is missing a tooth.
He is also a lion head if that makes any difference...?

Here is what he takes in on a regular basis:

He is supplied with 16 oz. of Oxbow Organic Rabbit Pellets and eats 2-6 oz. every day.
He is supplied with 1 tsp. of a fruit, 1 tbs. of a non-leafy vegetable, and 1/2 cup of leafy vegetable and eats all of it every day.
He is supplied with an unlimited amount of a hay mixture (3 parts alfalfa, 1 part orchard grass) and eats about 1 1/2 cups every day.
He is supplied with 4 yogurt chip treats and eats all 4 every day.
He is supplied with 30 oz. of fresh, cold water and drinks about 4-20 oz. every day.

Is this the normal intake of a senior rabbit? He maintains his weight just fine. He is not fat, nor skinny. He weighs about 4 lbs and is generally a very small rabbit.

I think for senior rabbits, reduce the veggies a bit and increase hay. Absolutely NO yogurt drops from now on. Rabbits are lactose intolerant and develop bad eating habits when they're given sugary treats. Just stop feeding them. If you want to give him treats make your own. (recipe below). You need to increase his roughage to reduce the chances of negative consequences of a slower metabolism. Unlimited hay, if he doesn't empty his hay rack(s) then skip a pellet meal and add more hay. You can give him hay in a box (often called a digging, or foraging box), stuff it in toilet paper roll etc...

Recipe for rabbit treats:

*1 cup rabbit pellets
*1 cup oats
*1 tbsp honey
*3-4 tbsp olive oil
*handful of blueberries, half a banana or half a pureed carrot *optional
=Preheat oven to 350F. Mix the pellets and oats in a food processor until powdery, add honey, oil and fruit and mix up until one greasy ball. ROll out on a board until thin (cm thick is definitely maximum). Bake for 15-20 minutes, halfway through break the 'dough' in half so it cooks throughout. Leave in the oven to cool for an hour or more. Then break them up into small enough pieces. Since this yields a lot for one bunny, you can freeze half of them.

***eta- instead of oats you can add alfalfa pieces (you know, the pieces at the bottom of the bag.. Magic Bullet (food prcess) them into a powder with the pellets in order for them to stick.