We switched to Royal Canin several years ago because Lucky had intestinal 'problems' and I was tired of him feeling bad, cleaning the floor, and vet bills (when the dog has diarrhea the health dept wants tests in case it's something that can be transferred to humans and it was a fast $180. every time). Anyhow, it cured Lucky's tummy troubles. We stuck with it and the two we have now have beautiful coats and are healthy. They make all kinds of dog food varieties including a couple diet ones. 5 lbs isn't a lot and it should be easy to just cut back a bit on the food and treats temporarily.
Years ago we had a pit and his vet kept pushing diet stuff. He was NOT overweight in our opinion. At one point he gave us a video. When we saw him again he asked about the video and we told him the dog didn't show any interest in watching it. He didn't try to get us to buy diet dog food after that.