any suggestions for a low protein diet  food

Forget the Diet bit, just feed a little less.
If you feed complete then look at one of the "Maintenance" grade foods aimed at working gundogs such as Skinners Field and Trial or Ruff and Ready. It is cheaper because it is vat free. I would recomend Masters Meaty Crunch which is what I use during the closed season but it is rather harder to find. During the shooting season when my dogs are working hard then I change to a higher protein "Performance" grade food.

Regards, John

If you insist on lowering protein try Autarky senior. Its about 19% if i remember correctly, i dont have any senior citizens with me just yet. I have used a senior food to try and slim down a dog before and it worked with a little extra exercise, maby an extra 30 mins playing fetch (or i throw, you chase, i collect and we start again as the case is with my Akitas) and an extra 20 mins on each walk providing the dog is well over 18 months.
Hope that helps,
GM

JWB senior/light has the lowest protein I could find. I didn't switch to it for weight loss purposes tho, and he (active 3 year old) hasn't lost any weight, despite being fed slightly less then the reccomendation. So I can't personally reccomend it for helping with weight loss!

A low protein food which my vet advised me to feed one of my dogs is
tinned chappie!

It only has 5% protein.
I know it's not the most wonderful when you read ingredients listed on the tin.
But it has the lowest protein content I've come across.
Whether it would be considered a diet food however is another matter. :)

Chappie is low in protein Boxacrazy but you need to look at the food it's self. Chappie is a tinned food and as such has quite a high water content whereas something like a dried complete food has negligible water content so will always have a higher protein level. To make a direct comparison you need to know the level with all the water removed. At that point it would be very little different to the complete's mentioned on here.

Regards, John