I remember the days when eating was a chore/war. Its very VERY difficult to gain weight as you know. IT's also a very difficult when gaining that 1-5lbs is very much OUT OF our control. when I got my G-tube in place I gained 10lbs immediately. but that was it. for about 5 yrs until I hit puberty and gained another 10lbs. you have to realize as you probably already do, its out of your control , for most CFers. you force and force yourself to eat and you gain maybe 2lbs. thats it. I used my tube everynight, 1500calories. if I missed two days in a row i would lose 2 lbs. it was that simple. When I was very sick sometimes I would do 24hr tube feeds. here are some tips.
Scandi shakes/ or 2 boost for breakfast before school (along with a bowl a cereal)
2 boost with a normal lunch
for school if he doesn't feel he is hungry provide him with high calorie snacks like cheese and meats and crackers, or trail mixes with chocolate ect. you have to get creative
A scandi / nutrishure pudding for bedtime snack
and ALWAYS have some in the house, so if your out of he gets the slightest itch for a snatch its there and Easy and quick!
And don't punish him for not eating, give him alternatives and choices, he will learn very quickly that an extra few lbs will give him leeway when he isn't eating. and if he doesn't eat his body won't have energy to fight off infection and he will miss out on xboxing and friends!