I was in a similar position to you (though thankfully without the teen porn.)
I found out when I was on holiday with him and our children. I didn't want to ruin the children's holiday. We were both on holiday until September and I didn't want to confront him and have us both in the house 24/7 together. I decided to tell him the night before he went back to work. I spent those weeks planning how things would work out and getting used to the fact that I'd be divorcing. After a long time together, that takes a lot of getting used to. He was completely oblivious to all of this.
I think there will always be some reason why it can't happen now - as someone above said. Once your younger child goes to university, the other may return. However, in your position I wouldn't do anything until the AS exams are over, if you think your child will be distraught. Honestly, though, the summer holidays would be long enough for your children to get used to the fact that things will be different.
Given there are only 18 months until your youngest goes to university, I would say that you and your child/ren should remain in the house for that period of time.
I would consider getting back in touch with the friend you lost who couldn't cope with you staying with him. She will probably be your best support.
In the meantime, take care of yourself. It is incredibly stressful to live like this at the moment and you really, really need to share the burden with a friend.
I wouldn't confront him until you are ready for him to go. The atmosphere might be bad now, but it will be terrible if you tell him you want him to stay for a few months but eventually leave.
One thing you could do is to tell him you have seen the teenage porn sites and you will call the police if you ever see them again.