N.
My two youngest kids, 8 & 3, prefer baths most definitely, but will take showers as well when time is short. I think it doesn't matter as long as they are getting clean somehow. As far as bathing at someone else's house goes, unless they get unusually dirty, or are there for an extended period of time (more than a 1 night stay), I don't think it's necessary for them to bathe at the other person's house. And unless the place he's staying doesn't have a bathtub (they only have a shower), I can't imagine anyone would have a problem with him taking a bath instead of a shower if he did need to bathe.
I guess you should just prepare him by explaining that when he's home, baths are fine, but if he ever has to bathe at a friend's or relative's house, or if he ever goes to camp, they may not have a bathtub and he may need to shower instead. Hopefully at age 9, he can understand that.
I think kids preferring baths is perfectly normal. I know both my daughter and son don't like the spray of the shower hitting them in the face and that's why they prefer the bath. Like I said, they will shower if necessary, but their preference for the bath is perfectly normal. I personally prefer a nice relaxing bath over a shower any day. :-)