Try to prepare a plan ahead of time for when he's out of town. Do it sober and take your time! Since you know you'll be drinking, you also need water (+ an efficient and easy way to get it and to remember drinking it, since we tend to forget to drink water when alcohol is involved!), a safe electrolytes drink or supplement, and at least one safe food to eat before you start drinking. Something that will be good for you and won't trigger you.
Plan it carefully, take your time, and try to remember that this is a safe plan you can rely on and will protect you from feeling ill when drinking.

Water + electrolytes are also very important if you're restricting or when you're feeling ill. Even if you didn't drink.

I started doing that plan because I'm a heavy vodka drinker at home. I knew that drunk me doesn't remember about water, and hungover me or weak me can't physically go grab water in the kitchen, so I got a big 2L water bottle that's always near me and within my sight (easy to reach, easy to remember if I see it), never forgetting to fill it to the max before I start drinking.
And I decided on a safe food that I'll have before I start, even if it's just a little bit, it's better than nothing.

The electrolytes part, I learned my lesson after I had several different restriction days where I found myself feeling so many terrible physical symptoms and I was seriously scared. I used to think that drinking lots of water would keep me safe and hydrated and that it would be the only thing necessary, but it's only half of it.

Take care! ❤️ I hope you feel better soon!