Any sort of fat (except the kind that come in processed foods) is healthy for babies/toddlers. Milk fat, so in butter and full fat dairy (whole milk, yogurt), is great. You can add butter into lots of things that you probably aren't already, like baked beans, topping for veg (melt butter over carrots or peas or broccoli), etc. Bananas also have healthy fats in them, as do avocados, if you want plant fats. Any sort of nut butter is great too. All of them will be great for a toddler, as humans get fat because they eat too many calories, not because they have too much fat in their diets. So you don't have to worry about making him fat, especially if he's needing to gain a bit of weight anyway.