Bogeyface. For a short course of pred, you don't need a reducing dose. They have quite a long half-life, so it will take a day or so for him to come down to earth properly, after the final dose, then he may feel very tired and out of sorts.
I have regular courses of steroids due to asthma. The first course I took, gave me a steroid psychosis. we'd just moved house, and I was convinced the fridge was sending me messages from our new neighbours. I ended up in the police station at 11pm, waving a bit of the fridge around, shouting, and still in my pyjamas - they had to get the MH team out to sedate me 
I can still remember the desk sergeants face, and it was a long time before I could go in the fridge without feeling a vague sense of unease 
Second course I took, I was in resus in A&E, so they whacked me full of hydrocortisone, admitted me and started me on pred. There was no other choice, because I was so poorly but because I was in hospital, I was still ramped up and revving, but I got support and medication to help me sleep.
I've had several courses since then. I've never had another psychosis like that first one. In part, it was due to not knowing what was happening to me, and not getting enough rest - it was really frightening!
Now, when I need steroids, they also prescribe something to help me sleep at night, cos even some rest helps break the cycle, and makes it more manageable. I go through my diary, cross everything out, I keep a low profile, but invite friends round for company. My children also pick up the slack a bit, and don't invite a cast of thousands round for tea. I let all my friends, ex-dh etc know I am on steroids, and they all keep an eye out for me. I also have the number of the MH team, so I can ring them, let them know, and we keep in telephone contact for the duration.
For me, I don't sleep, I feel like I can't stop moving, it is very difficult to hold onto my thoughts - like trying to nail a jelly to a wall - but with a bit of sleep, support etc it's not too bad. And now I know what to expect, it makes it so much easier.
Some people do have extreme mental health reactions to steroids, and every time can be different. If you look at the info leaflet that comes in the box, there is a big warning about this. It would certainly be worth chatting to the doctor about this - depending on how he is, it might be possible to rearrange dosing, maybe prescribe something to help counteract the side effects etc etc.
please do go and talk to your doctor about it - if you want to PM me thats also fine!