We raw on a mission to bulk up ds2 also....ideas from my gp (mainly covered above)
Adding milk powder to things - including drinks of milk - so it's v creamy!
No juice at all - they fill up on it and it's not adding bulk - milk or water w meals
Nut butters
Avocado
Stir olive oil thru pasta before sauce
Add cheese to things

We have also been using incremin which boosts vit and iron and also seems to help appetite ( he has been sick a bit)

The dr told me to lay off really seeded breads etc - the bulk makes them feel full.... I do use wholemeal but not using really seeded breads just at the moment

Al, of the above have helped us

Some things I have made tat work well...
Smoothie w milk, milk powered,honey, nut butter , an egg and banana ( and some yoghurt usually)
Pancakes w added milk powder and homemade lemon curd ( heaps of egg yolks and butter)
Macaroni w cheese sauce and tuna and corn - added olive oil and milk powder
Home made hummus - pursed chick peas, tahini and olive oil... Dip crackers in ( ds 2 will even dip cheese in!!)
Cheese on toast - also can dip into hummus
Chicken schnitzels - add olive oil to egg when crumbing