Reliantrobin, my dd2 had her tonsils out this year (she's also 4). I think what is important is that people had said to her 'once you have your tonsils out you won't have a sore throat any more'. Well, of course when they wake up, they have a terrible sore throat!! I think that needs preparing for, but it doesn't last for too long.
Also, be prepared for him to need pain relief for a week or more, I think they generally feel 'below par'.
Make sure you have plenty of supplies of ice cream, yoghurt, ice lollies etc.
There is a great book you can get on amazon called 'Goodbye Tonsils' which we read a few times, although it's about an American child so some of the terminology needs adjusting if you're in the UK.
My dd2 was quite sick afterwards, and don't be alarmed if it looks 'bloody' as lots of blood can be swallowed apparently.
I can't think what else is helpful, just be prepared for him to feel a bit sorry for himself.
Oh, just remembered, our dd2 was offered (and went on) a tour of the unit the week before , so saw the anaesthetic room etc (even though she had surgery there before) I think that helped.
I hope that helps.
BTW, dd2 was like a different child after, she could eat, speak and do all sorts of things she hadn't done before, and is free of monthly antibios!!