Try supplements, protein bars and shakes, if you increase your protein and calorie intake you should see a gain in weight. You will need 1g per kg of body weight minimum so approx 80g of protein per day split between max intake of 20g per sitting. I would say expect to see a 1Kg increase per month. Continue to train the way you are comfortable with for strength to avoid injury. 8 to 10 reps, 4 sets, gradually increase the weight over time....remember this hard work or everybody would be doing it