Help on dog foods, treats?

I've been giving my dog (3 yr old Chihuahua) Castor & Pollux Organix dry food with a spoon full of wet food from Castor & Pollux Organix. I'm trying to get him on a diet, poor thing is slightly over weight according to the vet. I don't want him to have health problems so I went to our local pet store and they suggested switching him to a grain free product. We purchased a bag and some wet food from Wellness Core which says to be grain free. I wanted to know if anyone has used this product and if so, was it good? My other problem is treats, I limit his treat intake to games we play but my mother tends to over treat him. When we play games I juse Bil Jac treats, I also have Dogswell treats which my mother tends to give to him without me knowing. I've tried to hide the treats even not buying them but my mother doesn't listen. She also tends to give him potato chips, pretzels and other junk foods. How do I get her on the same page? I try to talk to her and explain dogs are not like humans, their stomach is smaller then ours but it doesn't work. I need help...