I'm a life long healthy skinny, but if I accidentally lose any ( illness, stress, v busy etc) I always make an effort to regain it for next time I need it.
I've found fat is the best way. Excess carbs above appetite level I just end up with loads of energy, but fat makes me more lethargic, and takes up less space so I don't feel full as easy. Loads of cheese, fried bread over toast, cream heavy sauces on pasta, in casseroles, to cook in etc, gravy where you can, cheese sauce on meals, crumpets dripping butter topped with cheese over sandwiches, leaving veg off the plate at meal times so you eat more high calorie foods, then remembering to eat them when you have room, roasting meat over grilling, leaving bacon rind/ chicken skin on etc.
Full fat milk is good too, but I hate the stuff.
And while I wouldn't recommend adding loads of sugary crap, make sure he is getting enough for normal use, preferably from non filling sources like fresh orange juice and complex carbs.
I also find it easier to eat more than I want if there are few distractions, I imagine it's similar for a child. I can think of a million better things to do than eat when I'm not hungry, and I suspect he's similar. Mindless eating in the car is easier than at a sociable meal etc.
I would really only attempt any weight gain on a child with medical advice, some kids are just stick thin and perfectly fine as they are.