Yes definately dont take the x mass, it wont help you if you cant gain weight now. It will basically give a 25 year old the test levels that you have right now.
18 is the best time to grow. If you have a fast metabolism (like me), the only way is to keep constant pressure on your body to eat full meals every 2.5 hours. It wont happen right away, but if you dont give up, your body will start to react to the pressure, and say "hey, I need to start doing something will all this food he keeps giving me".
Get 2 kinds of protein. A weight gainer, and an isolate. You need the isolate especially for after the gym, because it is fast absorbing.
meal 1 - in the morning, take the weight gainer with your cereal (in the same bowl).
meal 2 (halfway between breakfast and lunch) - big sandwich (sprouted grain, meat and cheese), and a bannana.
Meal 3 (lunch) - chicken and rice and salad.
meal 4 (pre workout) drink your isolate and eat a bowl of rice.
Meal 5 (post workout) immediatly after your workout (at the gym) mix in appx. 1/4 cup of dextrose with your isolate. drink. The high GI carbs such as dextrose will spike insulin increasing the protein uptake of your muscle and overall anabolism.
Meal 6 (dinner) chicken, rice, salad.
Meal 7 (before bed) take your weight gainer.
chicken and rice can be any meat or carb. Brown rice is the best. Potatoes also work nicely. Beans as well if you dont mind the gas.
Dont have tiny snacks. Your snacks must be meals. Do this and you will gain weight like a mofo.
Especially if you train like this.
appx. 1 hour. 4-8 reps, 5 sets, 20 secs between sets.
monday - chest, shoulders
tuesday - back
thursday - legs
friday - arms
Try it out. let me know how it goes.