Back in the 1980's I worked out a lot to gain weight. I suggest that you do a lot of leg, chest and back weight lifting. Eat Protein and do at least 8 repetitions of each exercise, move up in weight when you can do three sets of 12 at a given weight. I'm sure I'll get corrected if my memory is wrong, but for size, to high repetitions, for strength, do low repetitions.
Squats are the best exercise for gaining leg strength. Bench pressing or pushups are good for you chest, and rowing exercises are the best for your back.
Hope this helps.