Thanks for your replies.
As for the nightlight, he does have a night on at night in the passage just outside his room and his door is always open. I might try and put another in his room for a bit extra light.

We've done some drawing (although not specifically flood related), and we did head out to the massive lake which is spilling over for the first time since '95 and we showed him the water and he didn't seem scared at all.

I will try some of your suggestions santosha. I think I'll talk to my MCHN aswell and see what she suggests.