Eat clean 5-6x a day slightly above maintenance level calories (how much is different for everyone - 500 is a number I see thrown out there a lot) never skip breakfast again, get a trainer and/or a copy of Rippetoe's Starting Strength and engage in progressive resistance training - squats, deadlifts, pullups, and bench presses, and sleep at least 8 hours a day (teens might require more sleep, not sure!). Rinse & repeat. - DaleK

Gaining weight is sometimes much harder than losing it . But it can be done. Make sure no health issues. Dales Suggestions of - never skip breakfast, eat more meals with a higher calorie content, sleep are all good. Best advice is a 1x visit to a nutritionist who have worked with ppl that need to gain wt. ( call and make sure to ask if they have worked w/males needing to gain wt) Aim to gain weight gradually with healthier foods versus high calorie sugar and saturated fat items ( candy, fries, processed foods, soda). Still eat fruits & veggies. Smoothies, PB, Lean proteins all good. Be patient which is a horrible thing to say , but true. Lifting is good , but make sure you learn correct form, function & have a decent plan. (tons of good books (library ) on both nutrition & lifting. Just a few ideas, but not a nutritionist ( work with tons who would kill me if I gave advice.) Good luck. fyi; best friend who is 6.ft. left HS under 130pds ( 58kg) and within 1-2 yr. he gained 35 pds by just eating healthy - Tracy Benham

Thanks for the tips guys! Much appreciated. One more thing I forgot to mention: I'm allergic to gluten (wheat/most grains basically). I've heard that being unable to eat these types of food is going to make it significantly more difficult to put on weight. Is that true? - David Adam

more challenging - another reason to see a nutritionist - one with a real degree - not just a certification. (even if it is 1x) - Tracy Benham