We had the same problem with my now 2 year old.
Avocado, egg yolk, if he is sensitive to dairy, they can "butter" stuff with Smart Balance, quesadillas if he can have dairy, coconut milk. Once he turns 12 months, it will be easier since he can have whole eggs. Bacon, sausage. Noodles with olive oil, garlic and herbs or butter if he tolerates. Waffles, pancakes, french toast. Also quick breads like banana bread, etc.
We used a homemade rice formula mix that had fish and flax seed oil - if interested in the recipe, send me a private message. Otherwise, they can give him a 1/2 t of fish oil a day and 1t of flax oil a day in whatever he drinks. Once he could tolerate dairy, we mixed cream into his milk.
If you google "high calorie foods for infants" or "high calorie foods for toddlers" you can get some good pages.