Please tell us you are using condoms with him and have been tested for STDs, because his behavior is very risky TO YOU.
It is clear he isn't going to be honest with you about it. He has lied so far. He has covered his tracks by deleting history and, you know of one email account, he could have made more.
Its all in what you are willing to put up with. So far your husband knows you are willing to live with things the way they are, so he doesn't have to change. He just has to cover his tracks a little more heavily. No problem. He'll lie a little more.
You can say when enough is enough.