Reason: Moved to weight gain
Eat calorie densed foods. Sweet potatoes, potatoes, whole wheat pastas, rice, beans, orange juice, fruits, avocados, nuts, nut butters. I think it won't hurt if you start with at least 2,500 cals per day knowing you're a teen and really need to gain a good amount of weight.
Eggs, full-fat dairy, olive oil (for cooking or as a salad dressing). There are lots of healthy, calorie-dense, foods.
Here is an older thread on Calorie Count that gives some additional ideas: