The same thing happend to a friend of mine when he was younger. His mom caught him watching gay porn but he denied that he was gay, he's 21 now and still couldn't tell his parents he's gay (even though it's quite evident). He even had two boyfriends already and hasn't come out to his parents. What I would suggest is to tell him that it is okay if he doesn't want to talk about that but it is okay to you if he is gay, bissexual or wathever, you love him regardless.
For me the thing that stopped me from telling my mom for a long time was the fear of her feeling ashamed, sad, making her feel like she had failed as a mother, or even worse, not loving me anymore. Even when I already knew it was ok to be who I am, to not know if it was ok to her was really hard. My mom is 100% ok with it but if she had made it clear the she was ok when I was younger, maybe it would mean I wouldn't have to face the 6 years (or more) of pain I had since I realized I wasn't straight until I came out to her. Regardless of what sexual orientation he has, he should feel you will love him just as much you would if he was to be straight. Even if it takes some time for him to accept himelf and figure everything out, he will know he doesn't have to fear to come out because he will know you are ok with him being like he is.
When I was younger and started watching gay porn I genuinely thought it was just a fetish at first lol so it's quite possible that he isn't lying and just hasn't figured it all out, or he may be in denial, or he is afraid. So well, 3 important things. 1st:tell him you are ok with it and make your home is a supportive place (it may already be, but make sure he knows it is a supportive place in relation to sexual diversity as well) 2nd:don't force him to confess 3rd:give him time to figure it all out, it may take weeks, months, years, who knows.
I would always recommend therapy, regardless of anything (but not a gay cure therapy, for the love of god lol). But I think if you recommend a therapy now to him he may feel threatened and get defensive.
Remember that in school it can be quite hardcore for a lgbt teenager, even if the person isn't bullied directly the fear and the struggle is real, so it may be a great relief for him to know that he doesn't have to be afraid at home and that he is safe at home
I wish all the luck for you and your son!