In order to gain weight, meals should be eaten every 1-2 hour and thirty minutes to two hours. Keeping a constant flow of nutrients in the body is essential for gaining mass, and you should have at least six meals a day to ensure he is getting enough nutrients for your body to grow.
These are something that will just help you get his calories in, supplement such as True mass, Serious mass and Pro gain can do, but pay attention, these thing's should be eaten in moderation because it will do the body more harm than good since they aren't nearly as good as real food, you shoud get his needs of protein, carbs and fat from natural food as a primary sources
you should ask your Personal Trainer to give you the most suitable supplement for your body. and ask him for the right technique for the exercises to gain muscles mass.
I'll give you a high calorie- protein diet, follow it as a first stage
.In the morning on an empty stomach : 1 cups whole milk+ 1 scoop protein powder
After 1 hours ( breakfast ) : 3 slices of whole grain bread + 3 boiled egg or fried with 1tsp olive oil or 1 cup cheese or 6 tps of beans or soy with a squeeze of lemon and little of salt, vinegar, green onions + 1cup of vegetables
.After two hours : 1 cup avogado with hony and nuts
After two hours : 1 serving fruit ( apple / orange / grapefruit / guava / cup of strawberry) or 1cup of natural fruit juice + 70g cack or muffin
After two hours :300 checkin breast ( grilled or poached) or 300 grilled fish or 300g canned tuna( large size), or tray 5 slices of grilled steak or 5 slices of turkey(all kinds of meat, combined with the lemon juice and all the wanted spices) or 1,1/2 cup of lentils or Chick Peas( added cumin and lemon ) or 2cup green bean or white beans +1 cups of green salad (add tsp of olive oil) + 8 tps of rice (cooked) or pasta or corn or 2 slices of whole grain bread
.After two hours: protein shake + 1 fruit serving
After two hours ( dinner , preferably before 3 hours from bed time ) :1 cup yogurt +3 pieces of whole grain biscuits or 1 cup of protein sald( 4 tps of red kidney bean + 50g feta chees+ lettus+ tomato)+ 1 cup mashed potato or 3 tps of oat mixed with 1 cup milk