E.P.
My kids were always funny that way, too. Has this caused a problem? If you are going to watch him for an ongoing time, I probably wouldn't worry about it. He may also be more on a schedule to even go once a day (when home). Offering him a fruit-type snack, like grapes, peaches, etc... at a strategic time , can be helpful.
My children's decision to "go or not go" at someone's home, usually involved either: Lack of fan, location of bathroom to other parts of home ("Can people hear me????"), a poor lock, or dirty bathroom. I could NEVER get my kids to go to a gas station bathroom in an emergency, when they were young!
Funny thing is ... With one of my good friends, we used to laugh about this. Whenever my children and I would go to visit, my daughter "had" to go to the bathroom, immediately. Pretty smells, candles, a nice picture, and good privacy, in my friend's bathroom was just what my daughter desired!