Image by PublicDomainPictures, Public Domain License, Pixabay
For book lovers you can get all sorts of books or even a kindle or a tablet or simply kindle book.
Music lovers will probably love CD with their favorite group or maybe something else with his favorite group if he is really a fan like a calendar or T-shirt or something.
For movie lovers you can get Blue-ray player (they can be pretty inexpensive) or just a movie or two.
For gadget lovers there are always new tech toys available.
If your teenager boy loves science and astronomy, here is another idea - how about a poster with space picture (galaxy, stars,...). Below is one of the posters from Zazzle - Pleiades Poster. You can get small posters (from 6 inches by 4 inches) or extra large one (50 inches by 40 inches) and all between.
You can find Pleiades Poster at Zazzle.com
If boys love to create and build, here are always Lego.There are a lot of great sets for 15 years old boys and adults, like for instance LEGO Architecture, Lego Creator, Lego Star Wars sets for 16 - 20 year old. And 3D puzzles can be great fun too.
I believe that the key of buying gifts for teenage boys is as simple as possible, so let's not make it to complicated at the very beginning already.
I wish you good luck with the gifts and Merry Christmas.