Male neutered cat keeps mounting unneutered female cat

Before anyone says anything she’s due to be spayed in about 5 days but unfortunately she’s gone into heat a couple of days ago.

It seems like he’s responding to her mating behaviour and keeps mounting her when she’s in mating position. I was under the impression that most neutered male cats would ignore this kind of thing. He is an older rescue, I’ve had him about 6 months and he is 12 years old so I’m unsure on what age he was spayed.

They both seem to be reasonably happy, not stressed or aggressive. I have been keeping them separated at night when I can’t keep an eye on them. Is it super important that I discourage him when it does happen? Is there anything else I should be doing during this time period?