Hmmm , if only there was some kind of additive that would give your brush extra glide which almost eliminates any risk of scratching glass........

On a serious note it's extremely unlikely that you've scratched his windows. It is possible, but very unlikely.
I've only ever scratched glass once and that was when I was using a razor blade, you hear the noise instantly and you realise that you've done it.
Unless you've been using the brush stock to scrape off bird mess, that can also scratch the glass.
My advise is to what's been previously said about giving him the brush head and saying to try and scratch any glass, if it does scratch you will replace it. This will show him how unlikely it is that it's been you.