If 80% of his history is porn, and he even looked it up while using your computer, those are signs of an addiction. If I'm right about that and it is an addiction, he won't be able to stop himself just because he's in a relationship - that's the nature of addiction.

If it bothers you a lot (seems like it does since your 'heart dropped' when you saw it) you have to decide whether to talk to him about it or just end the relationship.

If you talk to him about it there's no guarantee how he'll react, but even if he's honest and willing to try to quit (the best case) it still won't be easy. So it's up to you how much time and trouble you want to go through to work on this relationship.