Need to help a disabled Aussie lose some weight.

Question
About a month or two ago I adopted a 10 year old Aussie that has an injury in one of his hind feet that doesn't allow him to do much more than walk around the house and go outside to eliminate.  He is about 75lbs and visibly has a significant pot-belly, due to his age and his injury I don't know what I can do other than dieting to help control his weight.  I currently feed him no more than two cups of Beneful a day and I need to know if that is too much.  Thank you for your help.

Answer
I know you bought it with your dog's best interest in mind (all those healthy looking veggies on the bag), but Beneful is not what I consider a healthy food. Most people are unaware of the ingredients in their pets' food, but if you want to become better educated, here's a list of what ingredients to choose or avoid in general:
http://www.dogfoodproject.com/index.php?page=betterproducts
The lady who writes "The Dog Food Project" also runs a consultation service (my clients who have used her were very pleased), which you can access here: http://www.betterdogcare.com/
I have not found her to be any more or less costly than the canine nutritionists who are local to my area.  If you have specific goals, such as weight loss, I'm sure she could be of help to you.
Keep in mind that an older dog with sedentary lifestyle may not need the manufacturer's recommended level of food.  
Buying a high end food may seem more costly (sticker shock here is $40-$50/40 lb. bag), but if you compare kCal/cup, you often find that the premium food provides twice as much nutrition by volume, and you are actually not spending that much more money per serving.  And, you usually will save on vet bills in the long run.  Think of what would happen to you if you ate McDonald's each day, versus eating normal portions of organic gourmet:-)
Foods that I have tried on my own dogs and been satisfied with:
Innova, California Naturals (I use Cal Nat for my old hound with arthritis), Canidae, Orijen, etc.
Example of a food I might try for an older, less active Aussie:
http://www.californianaturalpet.com/products/default.asp?id=5
And, store locator: http://www.californianaturalpet.com/where-to-buy/
Good luck!