Have you had him checked for a thyroid condition? This can muck with weight. And is not something checked at a basic physical. Although, for a working terrier breed like this, 30 minutes every other day may just not be enough for him, especially as you are trying to get him to lose weight, not just maintain a good weight.

I am not familiar with Science Diet specifically, but since he is a wire haired dog you should avoid starches and stick to animal proteins, if only because it helps prevent skin disorders like hot spots. But it should also help your dogs system work more efficiently, which can only help him.

I know the die hard raw people will say there is no good kibble, but raw is not for everyone, and there are some kibbles that have similar effects to raw (espeically when supplemented with raw bones for dental health). My dogs are on Orijen, which is 70% meat with no grains. I also hear good things about Legacy (http://www.horizonpetfood.com/products-legacy.asp) and my friend in rescue says it makes the stool look the same as a on a raw diet.

Good luck!