At 19 testosterone will be sky high whilest estrogen will be at a minimum -you dont need an AI OR A SERM.As far as i understand it its as you get older you develop more of the aromatase enzyme. From what ive read its possible for a 59 yr old man to have more estrogen than a 50 yr old woman.
The fact that you can eat and eat without out gaining weight is down to your metabolism and age .Maybe try creatine or a nutrient partitioning supplement such as neovar which has both. Personally i feel an AI/SERM or TEST boost is not what you need
Cut back on your routine, aerobics for example leave out of your routine .Concentrate on the main muscle groups, chest, back , legs,-bench press/ deadlifts/ squats. Dont bother with small muscle groups, biceps/triceps /calves. Try this for 3 months and see how you go .Train 3 days a week , get plenty of rest and eat well.Good luck