If it's just a friendly trip and he's not someone you're romantically interested in, I don't think there's a need to disclose. Especially if it's important to you that no one in your extended social circle finds out, I don't think it's wise to disclose.
The only similar situation I've been in was also an outdoorsy one, and the excuse I used then is one you could easily use tomorrow. I told the folks I was camping with that I had a stomach issue and that while I had taken my medication for it, I'd feel a lot better if we chose a camp site that had a bathroom nearby just in case. That way, if I had to pee, I could just mosey off to the bathroom under the guise of my fake diarrhea.
Practical advice - take a change of clothes in your bag in case yours get messed up, but also consider layering your shirts so that you won't have to change in the morning (wear 2 shirts on Friday, bottom one comes off in the morning so you're wearing the "fresher" one for the rest of the day). If you're wearing a binder, I'd say maybe add a tank top layer to mask the appearance of it just in case your shirt gets wet. And use the bathroom at every available opportunity so that having to pee won't be an issue.
Hope this helps!