I would tend to agree that junk food is probably the fastest and sometimes the cheapest way to gain. The main reason is that most junk food tends to be very calorie dense. but in order to gain this density it contains some ingredients that taken in large dose's is not very healthy.
The major key to gaining is to take in more calories than you burn. You could work out at a profesional athlete level but if you take in more than you burn you will gain. So my suggestion is to get into some regular exercise, figure out how many calories a day you are using. Then just take in an extra 1000 calories a day. You can get these from healthy and inexpensive sources. The bottom line is he will gain weight, just relax and give it some time. even a pound or pound and a half a week will really add up
50-60 pounds a year is a good pace and just think in a few years he will be huge and healthy.
15 years