I'm so sorry you are going through this. I'm glad you found this group - you will get a lot of support here.
It is probably just as well that you have some time before seing him and hearing his side of the story. His side, will, most likely, to deny, deny, deny - then minimize and trivialize what you saw....when you know full well this is just the tip of the ice-burg.
My advice is don't rush into forgiving him. Take your time - it will take a long time for you to know if you can trust him (years). Don't email him. If you think there is any chance that you will want to reconcile, you want him to tell you everything - to give you full access to all accounts, emails, social networks..... If you email him, he will delete everything from his computer before coming home. Don't give him that chance.
I would cancel the vacation and get your money back. If that is a problem, get travel insurance then cancel.
I'm so sorry - but he is clearly a scumbag and you now have clear evidence of premeditation to cheat