Weight gain problem. Not much sturdy!

I usually take all sorts of food in a week.Always keep seasonal fruits in my food menu and take fruits and green vegetables daily in moderate amount.. I eat egg,fish and meat(2/3 times a week) alternatively in a week.. Despite all of the above,my weight hardly increases,my legs and hands look lanky.. I have no allergy to any food and hardly get sick in a year Being a muslim I offer Namaz daily,so it provides a moderate exercise to my body But my overall strenth and stamina are less than my peers.. What is the most valuable suggestion for weight gain and what to do to look more fleshy..??? SPECIAL NOTE : 1) I noticed a slight belly swelling after eating,may be some gas accumulation 2) My father and brother also look slim and not much sturdy