Meal ideas for weight gain

My one year old is falling behind with his weight and his pediatrician is starting to get concerned. I'm so stressed out because he barely wants to eat anything. What are some meal ideas with high fat and calories I can give him so he gains more weight? I'm adding extra butter or oil to all his meals and I do give him mashed potatoes with cream and butter. What else can I do? I tried avocado but he doesn't like it. He mostly likes fruit and green vegetables, but they don't have much fat or calories.