If he was stopped because of sticker, there is a problem with that.
It he was stopped for other reasons, but was asked about firearms because of the sticker, there is no problem with that. The purpose of putting a bumper sticker on one’s is so that other will read it.
And not to make fun of the cop, but that obesity (easily morbid) is probably a greater risk on the job that any armed person, legal or not. Don’t cops have rules about this?
There is no justification whatever to ask ANYTHING about guns. There is no answer that a person can supply that would justify lack of a search if the officer is justified in searching.