Ok....my opinion is a little different (seems to be a pattern lately
).
The advice towards calorie dense foods is good for us as we consciously want to eat more and we fight the feeling where our bodies tell us we have eaten enough in order to gain weight. (those of us who have working feedback mechanisms )
I have a very lean son as well. 11yo. His body keeps him that way. Getting food in him is a challenge at times, but I have realized that he is just in tune with what his body is telling him.
So the getting him to gain weight thing, you have to understand how appetite works. Food high in fat have a GREAT response to triggering the feeling of satiation. A great way to see this is take 3 shots of olive oil and wait 15 minutes and see how you feel. You will find you feel incredibly full (since you just took in 4-500cal). Even though the volume is small, it does a great job of triggering the brain saying "I just ate something".
So sitting in front of him and making him eat a single gainer meal, may just result in him eating less throughout the day.
The approach should come from a different way.....
1)Increasing appetite naturally
2)Feeding foods that dont trigger a high satiety response...hence he will eat more without realizing it.
The first will take care of itself naturally if he starts training as he gets older. But short of that, you can trigger appetite with certain foods. Combination of salt and sweet have been known to do this. There is some study that shows our appetite suppression can go haywire when eating foods like this. This is why chocolate chip cookies taste so much better when made with a little salt. Other foods like icecream can trigger pleasure centers in our brains causing us to eat more than we need. This is one approach we have taken with our son. He is allowed to eat ice cream anytime he wants. (We buy all natural full cream and sugar).
The second is feeding foods that dont tell your body you ate anything. A great example of this is soda. Drink a soda and see how full you feel. Now eat 2 apples. Aprox same amount of carbs, but two totally different responses. They did a study with young overweight pre-diabetic kids. They substituted only one thing in their diet and observed without any other variables. They were instructed to replace the liquid sugar drinks they consumed with milk. So oz for oz they are aprox the same in calories. BUT milk has a tremendous signaling response to the brain telling it you ate something in contrast to soda. ALL of the kids who drank milk instead of soda lost weight. The reason was, the soda cals were not telling the brain they had consumed food. When replaced with milk, their appetites went down.
So basically you could try to add in a bunch of 'empty' cals in the form of sugar drinks, soda, fruit juices, gatorades....etc. This is one we have not done yet since we have had success just giving him more of the 'goodie' foods, but if it came to it, I would be letting him drink soda if it was what was needed to him to put on weight.
Sure, you could take a bunch of sugar and add it to anything, but rather than a daily struggle, we have looked to other avenues of success. He is starting to take interest in working out. He is very active and a soccer player. But I have explained to him how strength training can help his soccer game. We are going to start a little workout program with all body weight stuff. I am interested to see if that bumps his appetite up as well.
Hope that helps some.