I am a 17 year old teen with ADD, I have the apatite issue myself as well as a blood sugar issue( hypoglycemia aka low blood sugar) I find swapping to crunchy veggies or things heavier in fat and protein can help, but still result in not eating often. Ideally try to get more interesting tasting snacks and make a game of eating them. Anything new could work, but if they have liked similar things it can be better. I personally like spicy foods so I would always have something hot and new. Too much of one taste can make them feel nauseated if you try to force too much at once.