I definitely think it's ok to point out the observation that you didn't feel he was present in the room during the session, so that he is aware that his 'floating away' was obvious. It can be something he just flags in his mind and works through whatever he needs to with whomever he needs to. It almost sounds like a positive experience, to witness him be able to ground himself and come back out, in ways you too have done. It doesn't sound like it would be a poor discussion at all, but rathen enlightening maybe for both of you.

I'm always for talking about it.