| If you're losing weight then you're not getting enough calories. Men need around 2500 cal a day to maintain their weight. If you want to put on weight, you'll need to have more than this a day. So start eating more!
Of course, your body stores excess calories as fat. Which is bad. You want to put on muscle. which also weighs more than fat but takes up less space on your body. It also makes you look good. So you probably need to dramatically increase your protein intake (like, 3 chicken breasts a day, or shakes if you prefer), which combining that with some weight training. You'll build muscle, get heavier and get healthier. | 
