We kept DS on formula until he was about a year and a half 'cuz he wasn't too keen on eating table food. He's still a picky eater. Anyway, the dieticians at the hospital gave us a recipe for higher calorie formula -- more powder to water. Then as he got older, his doctor told us to add an ounce of cream to it -- that adds about 100 calories.
DS loved yogurt at that time and I found a whole milk one made by stonefield farms, they also make yobaby. Otherwise I'd find the one with the highest caloric content and add a dash of cream. He had texture issues and hated ground meat, but loved stage 2 mac & cheese and babyfood meat, so I'd add some salt and butter to those. He also liked those individual microwavable soups -- bean, brocolli cheese and stew. I'd chop up the chunks and he'd go to town.
He's still a picky eater, but he loves spagettios, hamburgers & fries, pancakes... And we always give him carnation instant breakfast made with whole milk and cream with every meal.