Hi H.
I think it's good that you will be getting a little break from your kids and you will be able to spend some time alone with your husband! Your son can still exclusively breastfeed even if he drinks from a bottle occasionally. "Breastfed" means he's getting breastmilk, no matter how it's delivered. There will come a time, probably soon, when it will be really handy for him to be able to take a bottle. It can be expressed breastmilk or (if you want or if your supply goes down) it can be formula. I breastfed my daughter until she was 10 months old. I never pumped breastmilk when I had to (or chose to) give her a bottle. She still nursed all the rest of the time. Even if your son only nurses once a day, and takes formula all the rest of the time, he is still reaping the benefits of breastmilk. Just remember that your sanity is pretty important too.
I know this doesn't answer your questions about your worries that he will or won't return to nursing when you return. I think since it will only be a couple of days, it will be okay. But I have a LOT of girlfriends who chose to only nurse their babies, and when their babies got older, like closer to one year, they actually couldn't leave them at home because the babies had never learned to use a bottle and eventually refused. That's just as much of a problem as a child who won't nurse. Well, I guess it's only a problem if you don't mind never having an evening for yourself! LOL
Hope you figure it all out. Blessings to you!