First off, it doesn't sound like he was actually looking at porn. He could have just been briefly tempted (or even just curious), before getting a grip on his senses. Your super-angry response to this makes me think that you haven't gotten over his previous affair. If you're holding that against him, keeping that resentment and anger alive, it's more likely you'll damage or strangle your relationship than help it out.
Also, this is hardly grounds for leaving him, as one responder mentioned. I mean, seriously Leilaii?
As for what to do, if you have a very open relationship with good communication I'd ask him if he was having a problem with porn, in a caring and supportive way. Let him know how you're feeling, and why you're so worked up over the idea of him looking at porn. If he admits to a problem, get him some help! Getting an accountability partner, special software for the computer, or just getting rid of all kinds of internet devices are a couple of ways of fighting porn.
If your husband says he's innocent, and doesn't give you any more cause for alarm, you should probably give him the benefit of the doubt! People rise or fall to meet your expectations. If you expect him to be decent and good, he'll rise to meet your expectations! If you expect him to be an adulterer, he more than likely fall to meet your expectations.
God be with you,
- Topher