"He claims they're not his and maybe it's because..."
He is gonna throw a whole lot of maybes at ya so you can take your pick which one sounds plausible enough to believe. Whichever one makes you pause to think over, he will jump on and elaborate. He's bamboozling you.
Of course he is lying. The reason why he is lying is because he KNOWS he broke his promise to you. He decided his need for porn was greater than his promise, so he went behind your back and did it.
It is just so ridiculous that you are even buying into it. It reminds me of what wives do when faced with evidence of their husband's cheating... they will begin to entertain any and ALL excuses and they dissect the Possibility of each one ... for example, if the wife finds a text on her husbands phone that says, "Sex was GREAT last night :D " the husband may say someone sent it to him by mistake! If she then finds two movie ticket stubs in his pocket for the Wednesday matinee, he will assure her he accidentally paid for 2 tickets. If she finds a condom in his other pocket, with two missing out of a box of three, he will say, without blinking an eye, that he's holding them for his friend.
You may not believe it, but all these examples have actually been asked on these boards... so... I know you are too close to the wood to see the forest, but honestly if you step back and look at the Big Picture, what do you think is the truth?