So the role play is so helpful in preparing them because you can tell them about what might happen and why.
You need to take the lead because they don't yet know what you are trying to communicate.
Start off with little things, 'the nurse wants to take your temperature can you hold this under your arm?' 'can you lift up your shirt so the dr can see xxx' praise, 'the dr thinks you need to have some medicine and stay at the pretend hospital for a few days, let's take the medicine and snuggle in the bed together'
Keep adding in details as the play progresss but slowly they need time to play out the scenarios.
Play out taking medicine and what it's for i.e. This medicine helps your body not to feel sore, this medicine helps your body to poo, this medicine helps your body to relax. Be honest but keep it simple.
I found she was happy to take medicine if I explained why she needed it and what it was doing.
Play out having magic cream and a sticker on, bandage over the top.
This magic cream helps your skin not to feel sore, we put the sticker on so it doesn't rub off. The bandage keeps your hand nice and safe.
What other things is he likely to face
Scans? Being hooked up to machines? The more you can find out about his recovery process the more thoroughly you can prepare. For planned admissions the before hand stuff is pretty irrelevant imo, at that point they are feeling relatively well (or at least normal for them) it's all an exciting adventure with new pjs, even the actual going under ga is such a small part of the experience that I wouldn't focus on it too much.
'The anaesthetist will give you a special medicine that puts you to sleep, while you're sleeping soundly the surgeon will help fix a part of your body that's not working quite right. Once they are all done you will wake up feeling a bit groggy and I will be waiting close by'
All of that can be played out but it's best done with toys so that they can see all the different roles.
As well as the medical stuff drop in things like, granny coming to visit, the day we had ice cream for breakfast, drawing pictures, getting new crayons, Make it clear that funny things happen at the hospital and that's fine, no big deal.
Don't underestimate their intelligence just because they are small and look like babies when they sleep doesn't mean they aren't taking it all in. Use the proper words for things and a wherever possible seek their consent for procedures.
Most importantly keep yourself calm. They take their cues from you, if you seem relaxed about this new and unusual thing it's probably fine.
If need be take a min away from them to lose your shit.
dont be afraid to ask questions. There is no reason to pretend you understood what they said if you didn't, keep getting them to explain until you understand. They are a normal human who learned this and you can too