Have you tried giving him a probiotic supplement?? We got the infant drops from
http://www.everidis.com/ (the ones used in the study of colicky babies)... and wow, what a difference it has made for DD.
Let's see, healthy fats... coconut (the "meat", oil, and milk), avocado, any nuts, salmon, and tuna come to mind.
You can make hummus...which is chickpeas, tahini (sesame seed butter), olive oil, lemon juice, garlic, and salt. The olive oil and tahini are both healthy fats. Most kids like it... and they can dip veggies, crackers, and bread in it.
Oh...egg yolks are very healthy fats. You could make a custard using eggs, coconut milk, and some sweetener.
Can make coconut milk rice pudding.