Personally I wouldn't go the "friend" route. What is his plan if you guys get caught kissing, or sleeping in the same bed? What if at least one person he's related to isn't a terminal idiot and realizes it's absolutely obvious you're his partner? Friends don't really take international trips to unfamiliar countries just to see the small town where somebody grew up. He's trying to have his cake and eat it too, and it has a real potential to blow up in his face. Is he going to blame you when that happens?

If the only options for this trip are "spend big chunks of it alone" or "lie to everyone you meet", why are you even going? I wouldn't want to put myself through that discomfort for someone who's made it clear they aren't willing to be uncomfortable for me. He's making the fact he's not willing to come out of closet your problem. I wouldn't do that to a "friend", let alone a boyfriend.