I posted the following in another thread a while back. My baseball player son wrestled at 140 pounds as a high school freshman (and he never had to worry about making weight in that class) and 215 pounds as a senior, so all the weight he gained was lean mass. He wrestled 160 as a sophomore, and 189 as junior, so the weight gain was steady throughout his high school career.
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Here's what worked for us:
1) A hot, hearty breakfast every day. No cold cereal, no breakfast bars, no pop tarts. Real food: eggs, potatoes, meat, toast, and juice. Vary the routine with breakfast burritos, omelets, chicken/gravy/biscuits, and other recipes.
2) Pack extra sandwiches and fruit for him to eat between classes and after school before practice/workout.
3) Normal lunch during school lunch period.
4) Big dinner waiting on the table the minute he gets home.
5) Peanut butter sandwiches or cottage cheese at bedtime.
Don't let him sleep in on the weekends. If he really needs some make up zzzzz's, wake him up at the usual schoolday time, make him eat his usual breakfast, then let him go back to bed.
Most kids who say they can't gain weight actually eat a lot less than they think they do. A typical day for these kids starts with a skipped or junkfood breakfast, a school lunch suitable for a small non-athlete as early as 10:00 or 11:00 in the morning, then nothing until after practice ends. Then they binge on fast food and other junk after practice because they've been starved for eight hours. They think they're eating a lot, when in fact they're just backloading useless calories. After-the-fact binging doesn't undo the damage of letting the body consume itself all day long.
Fill him up with good food when he gets up, give him a sensible snack every two or three hours all day long, and top him off again with something that has both fat and protein when he goes to bed. Do this every single day. It works, and it's much cheaper than fast food or supplements.
Another side benefit is breakfast becomes one of their special memories. I'd get up early, cook the breakfast and set their plates in the warm oven or on the stovetop under an overturned skillet. The boys loved stalking into the kitchen and peeking under the skillets or into the oven to find what feast has been prepared for them. When they grow up and move away, they treasure coming home and getting "Dad's breakfast" served to them again.
Best wishes.