^ there’s not really a ‘nice’ way to ask but imo this is a topic you need to be open and clear with, “have you been tested for STIs recently?” Or “here’s an email showing my last STI check came up clear, how about you?”
If he’s offended then he’s probably never asked or been asked that question before, and if that’s the case then you don’t want to have sex with him anyway 🙃