You know it's possible that his 'trainer' wasn't being literal and said something like "Kid you're like 1% bodyfat, we need to put some bulk on you." Maybe we're all seizing upon this detail of his post a bit much?
As for gaining weight, there is no magic number you should be at. It's largely sport and individual specific.
Unfortunately, a lot of kids think a McBurger is to help them put on weight. Quality food will yield you higher quality results and fast food isn't it. If you are concerned about the cost, hit the grocery store. I buy roasts at under $2.50/lb all the time. Brown rice costs almost nothing... you get the idea. BTW, might want to avoid deli lunchmeat as the cost per pound is generally much greater.
Calories in > Calories out = weight gain. Bottom line. Barring thyroid disfunction, it doesn't matter how old you are etc.