You'll want to add calories for the fellows, especially the 17-year-old, so they feel full. A few ideas that my ex used to do:
Add lots of oil to salad. Toss the salad so the oil coats the greens.
Add seeds to the salad.
Mix oils into yogurt. A tablespoon or so mixes in quite nicely with some vigorous stirring. If you have ground almonds, mix some of those in as well.
Use a LOT of tofu (he could eat an entire package, one of the cubes about 1.5" x 3" x 3" at one sitting).
Put any ground nuts and lots of chopped up fruit in the yogurt (dates, figs, etc.). The yogurt can make a good breakfast or dessert. To sweeten, honey or maple syrup. You can add grated ginger as well.
Thick soups with squash would be good and filling, too.
A few peppers in the food to give it spicy heat will also help get people feeling full.
Another dessert idea -- raw or baked apples. Calorie-rich, tasty add-ons would be any nut butter and honey.
Good luck!