Even though he's lost your trust, hopefully you can come up with some ways for him to earn it back. He had admitted what he was doing even if he felt cornered. Being angry at him won't be helpful to neither one of you. At his age it is not unusual to have curiosity like this. You've got to figure out what made you more upset- the fact that he was curios about this sort of thing or the fact that he did not tell you about it.
He may have felt odd telling his parent about his adolescent hormones raging and the curiosity that comes with that. Allow your anger to cool off and then try to talk to him about what you find important that he should have knowledge of at this age. He can resort to watching porn even at his friend's house or elsewhere. Let him know how he can earn the phone and computer back. Attempt to see his behavior from his point of view if you were a guy at age 15 with access to internet, TV, magazines, etc.
It would be important not to shame his curiosity. He may have been engaging in this behavior for quite some time before you found out what he was up to. The past is not as relevant as the present that you and him can create. If it is trust that you wanted, then talk to him about trust and try to understand him. He has to trust you as well.
Hello...your son is probably very embarrassed that you finally found out about his interest in porn. At least he was honest with you about what he was watching. However, a year and a half is a rather long time to have the images swirling around in his head. It is not emotionally healthy to view the images even if it was spurred by curiosity. Taking away his computer and i phone may only help the situation at home. Friends share images on their phones and even magazines are passed around amongst friends. You cannot control that.
Your son must develop the fortitude to stay away from incidents where porn is available. You can talk to him all about the women and men in porn and what watching porn so much can REALLY do to a person. His mind will become warped and his relationship with women when he gets older may be rather different than the norm. He may seek a woman like he has seen in the porn movies.
You may want to have him talk to a psychologist or a therapist about the issue if he will not open up to you. Maybe there is a much deeper issue involved that does not appear on the surface. A psychologist can assist your son in getting to the root of the problem.
The sessions would be confidential so that your son could develop a trusting relationship with a neutral person. It would be devastating if this porn issue became a true addiction. Please take into consideration seeking continuous professional assistance so that your son receives the help that he needs.
Thank you and have a great evening.