My son is very tall 6'2 and only weighs 135. He would like to gain some weight. Is there anything he can take to help. He eats very well, but gains no weight.

Sometimes it just takes some time to put on the extra weight but here's some ideas that might help. Don't eat junk food, it's just too high in transfats and saturated fats. Choose foods with Omega 3 fatty acids sources like Tuna, salmon, flax and walnuts. Protein sources like poultry, fish, lean meats, nuts, seeds, legumes, chickpeas, kidney beans, peanut butter, omelets filled with cheese and veggies, whole grain cereals, rice and pastas, and bread. Granola, bagels and cornbread. Honey and jam, soy protein powder, Cook with healthy unsaturated fats like olive or canola oil. Try veggies like avocados, olives, potatoes, peas, corn, squash, Starch veggies, Bean soups. Fruits like canned fruit in syrup, fruit nectars, and dried fruit. Use protein powders and nutritional supplemements like ensure or carnation instant breakfast drinks and drink them with each meal, along with taking vitamins and minerals every day. Eat 3 large meals, plus 2-3 large snacks every day. Eat larger portions at each meal.
Use milk instead of water in cereals, soups, sauces, mix cooked chop meat and wheat germ, nuts, beams or cheese in casseroles or omelete, pasta and side dishes. Add 300-500 calories a day more to your normal intake, So you really need to know how many calories he's eating now and then make the adjustments. And try to work in some resistance training, it helps build muscle, which is healthy and weighs more then your normal fat.