Does he just forget, or does he just figure they will be fine left outside? If he forgets, Amber's or Aly's suggestions are good.. if he just figures they will be fine that's a bit harder. In that case I think I would research/write up a list of all the things that could happen if they are left that long unsupervised, try to explain it is like leaving a toddler to play out in the yard for hours unsupervised.

They could get sick just eating too much grass for instance. Tommy once got into some moldy grass clippings from the compost bin outside, I was doing something and looked up to see him hoovering it down. I got him out of it, but it ended up making him so sick it really wrecked his bowels and he had to spend days at the vet and it gave me a horrible scare. I know others have had some problems just from eating too much grass as well, Sandra's Star had a problem a few weeks ago from that.

I hope you will be able to succeed in this, you probably worry yourself constantly when this is happening.