K.,
My daughter needed to gain weight as well but I wasn't asked to put her on formula. The doctor said to add butter where possible in her food and cook as much as possible in olive oil. We saw a feeding specialist and she suggested Carnation instant breakfast drink instead of pediasure, it has less sugar and is less expensive.
Other high fat foods:
AVOCADO - you can mash this with banana or put it in the blender with banana -this makes a pudding consistency and if you add a 1/2 block of tofu it is a mousse like consistency. You can also mash avocado's and pears and add chicken to it for a really nice meal. I put chicken in the mini chopper and can get it as fine as eraser shavings if necessary.
PASTINA - star shaped pasta. I cook this, drain it, put it back in the same pot, add an egg and a dab of butter and cook, stirring the whole time til the egg basically disappears. You can also add peas or carrots to this. I sometimes grate carrots and zucchini into pasta dishes. With leftover pastina you can make a cheese sauce of a Kraft single ripped up and put in a bowl with some milk. Put in the microwave for 20 or 25 seconds and stir til combined. You can put almost anything in this cheese sauce.
EGGS - we had a slice of cheese to scrammbled eggs.
BREAD - cinnamon raisin toast with cream cheese is really good. Brown bread with butter.
CHEESE - shredded cheese as well as Kraft singles or even the baby bell cheese is good. Look for middle cheddar or mozzarella -whole fat of course.
FRUIT SMOOTHIES - I use fruit anything they like and whole fat organic ice cream - vanilla. Mix in the blender and freeze for a 1/2 hour. This is also good when they are teething, as are frozen blueberries.
YOGURT - drinkable yogurt is good too. Stoneyfield farms has a low fat one but I recently saw in one store that Yobaby came out with a drinkable yogurt. The feeding specialist recommended giving my daughter yogurt everyday.
SUNFLOWER BUTTER - this is instead of peanut butter. It is low on the list of allergies and is even used in public schools as an alternative if a child won't eat the lunch of the day. I put this on wheat crackers, on a english muffin or in place of peanut butter in recipes.
TOFU - really good for protein. I add it to cream soups or in pasta sauce. I daughter doesn't really know it's there and it's really good for her.
WHEAT GERM - you can add this to a lot of things. Just add it to the portion you are serving that day. Do not mix into something then store for later - it seems to turn black and for me at least is very unappealing to eat. You can roll banana pieces in the wheat germ for easier pick up.
You didn't say how much your son has lost but my daughter was born in the 5% and is now below the 3%. However she gained 1 pound 10 ounces on this high fat diet in 3 months. The doctor was happy with that.
Hope this helps.
L. M