Not saying they are good calories, but I know plenty of huge guys that eat mcdonalds 3 times a day to get the extra cals in. If you need to consume 8,000 calories a day, especially at age 15/16, eating oats/rice/yams/the whole shmeal is just not even feasible..
I certainly do not recommend the mcdonalds, or any other fast food for that matter, but it is an option if hes an extremely hard gainer..