Up your calories to 5000-6000 a day,eat clean carbed foods,oats,fruits etc.Keep protein extremely high,try and consume at least 250grams a day if you aren't allready doing so...hell since you ahve such a fast metabolism,up the calories even more until you notice you are starting to put on mass,keep the weight training going,low reps/heavy weight.All that coupled together should start building lean mass.Eat every three hours.
Eat,eat,eat,weight train,take a break from any cardio you are doin,and eat even more.
