being thin is a good thing, you dont want to be big because then you will weigh a lot and have to fight naturally bigger dudes.... keep your weight down and you will be up against small dudes that are similar to you
Weight lifting and change up your diet if you want mass gains, if you want to improve your boxing skills just workout more on the bag/boxing gym. There isn't anything that can make you stronger(legally) it's all mental; keep working hard and stay dedicated to the sport. You can do it!