Looking at the two different foods there really isn't that much difference between the formulas, ingredients, ect. I posted them below...are you feeding him the "recomended" amount on the bag? Because they really are just that, a recomendation, each dog's system and metabolism is different and should be adjusted accordingly.
California Natural Lamb Meal & Rice
Guaranteed Analysis
Crude Protein (min) 21 %
Crude Fat (min) 11 %
Calorie Content:
3,990 kcal/kg (108 g = 1 cup)
430 kcal/cup (3.8 oz = 1 cup)
California Natural Lamb Meal & Rice-Low Fat
Guaranteed Analysis
Crude Protein (min) 21 %
Crude Fat (min) 7 %
Crude Fat (max) 9 %
Calorie Content:
3,490 kcal/kg (91 g = 1 cup)
317 kcal/cup (3.2 oz = 1 cup)
For example, I feed my dog taste of the wild, all three varieties on rotation. The Bison and Wetland formulas have 370cal/cup and 375cal/cup and 18% fat respectivly. Where as the Pacific Stream formula is 360cal/cup and 15% fat. She was on the PS formual all summer because we thought we were dealing with food allergies coupled with more exercise/outside time kept her trim on 1/2c per day. Now that I've rotated to the next flavor (Wetlands) she's gained about half a pound and is getting a bit squeshy, 15 calories doesn't seem much to us in human terms but to a 14lb dog 4% more calories is quite a bit. So the solution is to cut back her feed a bit or increase her exercise.
So I guess in my long windedness explaination, what I'm trying to say is he likes the food and is doing well on it there really isn't any need to switch him off it. Just decrease the calories and your going to decrease his weight. If he still seems hungry I know some people add plain un-salted canned green beans as a 'filler' to help their dog adjust to a smaller portion.
Also, keep in mind any treats you give him as those are calories too and sometimes they really add up fast.
PS-here is CN's feeding calculator put in the 'weight' your feeding too now and the weight you would like him to be and compare the two amounts. That may give you an idea of how much you should be feeding him.
http://www.californianaturalpet.com/tools/feeding.asp