How about giving him the peanut butter on apple slices or celery? That still tastes like peanut butter (but the added crunchy is a good contrast), and then you know he's at least getting some other food groups.
But really, unless it continues for weeks (months) on end, I wouldn't worry about it too too much. Just watch out for constipation with all that peanut butter!
As far as the visiting goes, I agree with the everything in moderation thought. If you're at someone's house, you can also limit the amount of junk that he's allowed to consume (ie, one cookie or a few chips, etc), so that he doesn't feel deprived (especially if others are eating the items in front of him), but he gets that there is a limit, too.
My DS is just a little bit older, and he gets on food kicks, too, where he only wants one particular thing all the time. We try to get the variety in, but only sometimes are we successful. I keep telling myself he'll grow out of it.