
Unfortunately you have already dismissed the best food for nutrition and weight control.
I would suggest the next best would be a grain free product. Many of the better makes are quiet expensive but because the quality is so good much less is needed for your dog.
Orijen, Acana, Millies Wolfheart,
The "Zoofast" website prices are very good and quiet a bit cheaper than many (if they have what you want in stock !)
Natures Menu make some fantastic grain free products which can come in a dry form, tinned or pouches and of course their frozen range of convenient foods.
Weight control usually depends on how much is fed at the front end, and as your exercise is very restricted less would be better, swap some kibble for some chopped fresh raw vegetables to fill the tummy.
Carrots, broccoli stalks, sprout stalks, outside cauliflower green pieces, the middle piece of cabbage leaves, are all crunchy and good for the teeth too. Out of date or wrinkly fruit will also fill the tummy. If your dog does not like the raw veg then just very lightly steam or blanch it.