Weight gain isn't a "works or doesn't work" kind of a thing. If he's human, he can gain weight. I'd recommend keeping a food log (even though it may be a pain) of everything he eats. I mean, like, write down everything. It could be that he's only eating 1 or 2 big meals each day, or if he eats more often, he may be underestimating his current calories.
The other thing I've seen people unwittingly do, is to take a weight gainer shake instead of a meal. That, obviously, cancels it out. If he's taking a high-calorie gainer, it needs to be in addition to his whole food diet, not instead of.
Does he work out, or have an active job, like construction or something similar? His daily calorie requirements might just be more than he's taking in.
Other than that, there's not much new I can tell you. Calorically-dense foods will be your friends: nuts, peanut butter, dried fruit, whole grain bread, full fat milk, a bunch of meat. There's nothing surprings about what he should be eating. I just want to know how much he actually is eating.