Help gaining weight after worms

Our 2-year-old King Charles spaniel recently had worms. We believe they were kidney worms given some of his symptoms. Took us a week to worry about his symptoms enough to go to the vet, then another two weeks to get into the vet, then another week for the dewormer to work. But over the course of those 4 weeks, he lost a significant amount of weight. Almost 5 lb. Since his treatment his energy has come back, and his coat is healthier. He's eating a regular amount of food, about 1-1.5 cup of kibble which we've been trying to supplement with rice after he's eaten that. It's been 2 weeks and we haven't seen much of a weight change. Only about a pound up. As long as we're consistent with this pound every week or two, that's okay. We just also know he's still healing up from the worms. So we just want to be really careful about making sure he's getting a healthy weight gain. The vet isn't very helpful. So any advice about weight gain or what we can do to help his general health right now would be appreciated.