yes, makes sense to ask that and also to find out if he's on PrEP, etc. If he doesn't trust Pfizer, does he take Viagra or PrEP? Science is science!
Maybe he's just busy but if he got offended I would simply move on. Is not like you ask him for his real name or anything too personal. Many escort announce in the ad they're vaccinated and some even often to show their card.