All you gotta do is EAT EAT EAT!! But that does not mean you have to eat all the fatty food to gain weight. Do eat but Healthy food. Get all the proteins/carbs from your food. Take well proportioned fruits in your diet per day. Have juices/water instead of carbonated drinks. Eat three meals a day and in between have healthy snacks like protein bars and shakes. Don’t bother taking gain weight pills crap, all waste of money. Just eat healthy and keep up with the exercise.
OMG, I have heard countless times that "when he gets hungry, he'll eat...its a control issue so dont interfere just let him get hungry" yadayadayada...one time I gave in to these opinions...the psychologist told me to go two weeks without interfering in any way at mealtime. WELL....my son went 5 days without eating (well, he ate less than 10 doritos and drank a little milk) and he lost almost 2 pounds which he could ill-afford to lose.
Some people have a big issue with eating...it usually has roots in physical troubles and takes a long time to overcome since eating has been a negative experience. Once I realized this, my son improved.
There is a term called "failure to thrive" and if you fall into this category, it means that you're malnourished. And should seek some sort of intervention to counter it and put on some weight. If you are just naturally thin but of a weight that does not cause medical concern then those who complain at you should be asked in a nice way to take-a-long-walk-on-a-short-dock.