Tell your vet to study dog nutrition a bit more.
As Vikinggirl pointed out - dogs rely mainly on protein for energy.
Protein content in a kibble bag is a PERCENTAGE. It's not a set AMOUNT. Percentage means, that for every serving of dog food, only X percent is protein the rest is something else and they all have to equal 100%. So, what's the other things that make up the 100%? Well, in the bag, you will see that Y percent is fat, Z percent is moisture, W percent is ash (minerals and what not), and V percent is fiber. But, if you notice all that is listed does not equal 100%. So, what is missing in the list? Well, you can assume it is CARBS because not all carbs is present in fiber. The government does not require dog food manufacturers to list Carbs %.
So, okay, all dog food people know that dogs are not efficient CARBS consumers just like they are not efficient plant protein consumers. Well, humans aren't efficient carb consumers either (hence, Atkins diet became popular) but we have a much higher tolerance to it than dogs do. So, it is pretty much a guarantee that a lower protein percentage on a bag of kibble means it has higher carbs. You can't get away from carbs in a kibble. You need the starch to make the dough that forms a kibble. But, you don't need 45+% of each serving to be carbs to form the dough. More than likely, the dog food company wants to make the kibble cheaper so they put in only a little meat (more expensive) and fill it up with high-carb things like potatoes, peas, grains, etc. to make the target calories.
Now, look at the Prey-Model Raw diet. X percent is protein. This has no relationship with the X percent protein in kibble. Why? Because what makes up a big chunk of a Prey-Model serving is Moisture (water). In kibble, moisture does not stray far from 10%. So, when a Prey-Model Raw diet says it only has 30% protein - well, that usually means close to 50% of it is water, not Carbs, and dogs are efficient water consumers.
So if you say, let's lower the protein PERCENTAGE in the kibble (means increase carbs) so the dog will lose weight... uh, you're trying to tell me that you are going to exchange efficient protein to inefficient carbs as your plan for a dog to lose weight? No, if you want your dog to lose weight, feed him less. That will lower his protein intake.
