I think what might be best is if you can spend some of the day thinking about what you need to know from him and what you need at the moment re: space, time. (It's okay that those needs will probably change, you do not owe him any answers about what will happen in the long term.) If you can do that thinking on the field trip then good.
Once you have a sense of what you need for the next night or two, you can tell him. If you need to be alone in the house, then he will have to go elsewhere. If you want him gone by the time you get home from the field trip, or if you want him to leave after dinner, you can tell him that.
I would make sure that you have access to money, that he can't transfer all your shared funds elsewhere.
When you are ready to hear about what happened and how he feels, you can tell him it's time for him to sit down and tell you the entire truth. I'm so sorry in advance that you will have to hear it.
I really think, other than making absolutely sure that you're financially secure for the short term, you can take this day by day. He can stay elsewhere for as long as you need. He can wait until you're ready to talk, he can wait until you're ready to make decisions.
Glad you can go to your sister's soon.