Your 15 year old son is showing signs of teenage depression. Over the last few months he has become plagued by feelings of worthlessness, making him extremely vulnerable to criticism, rejection, and failure.
He appears to speak a lot less than before, and he is pulling away from you physically as well as emotionally. Further, his social life has become non existent. He goes out very occasionally with the few friends that he has left.
More worryingly, you have recently discovered a semi automatic pistol under his bed with a suicide note addressed to 'The World that doesn't understand '. He plays the Smiths continually.
Don't panic. Asking too many questions about what is troubling him could open up a whole can of worms and remember long term therapy isn't cheap !.
Simply go up to your son's room and offer him the occasional cup of tea to show him how much you care.
Also think of the money that you are saving. A 'normal' socially active teenager can be a real ball breaker when it comes to financing their night's out I can tell you .
And as for him not talking much, well let's face it most teenagers talk a pack of old shite at the best of times.