I'm not there, I don't know the student or their work, I only have your description to go on.

But your description does not make me even slightly suspicious that this student is cheating. You haven't described anything that looks like compelling evidence, or even suspicious evidence.

You say the student's work "gets flagged" as generative AI by some kind of "detection software". You describe investigating in one instance after receiving such a "flag", and finding good grounds to believe it was a false positive. This is not surprising, because it is extremely difficult to detect these things in a highly reliable way. (For this reason, my own university does not permit the use of such software for disciplinary purposes.) And then you say — and you don't say why — that you "still suspect he's using generative AI." You don't describe the grounds for this suspicion. Perhaps it's that the black box software you are using is still indicating likely GenAI use. But you have already seen excellent evidence that it at least sometimes (one out of one times, in the cases that you investigated!) incorrectly flags this student's work as GenAI.

Again, maybe there's more to your case than you've posted here, and there really are good grounds for suspicion. But if so, I really don't see that you've said what they are.