My DS had this at exactly the same age and has ASD so - here goes for a long post....
The hospital will see you as a day surgery case, so that basically means you will be in a ward for the best part of the day. Presuming it's a children's hospital - they will either make an additional appt to show you round the day ward, or they will have weekly tours for soon to be patients. These are led by the play workers and will show you and DS everything - the ward, the play area, and give you an idea of how the day will run. There are always loads of toys and things in the wards, so that gets you over the first hurdle - they aren't scary places in terms of what your child sees.
On the day you'll have to fast, which can be a real PITA for some routine-focused kids. Ours loves taxis, so we took a taxi to the hospital, and the excitment of that got him over the stress of not having breakfast IYSWIM.
Now. My DS at first had the partial snip where they basically snip the two tendons (?) at the side of the shaft which were causing the foreskin to stick. This particular op leaves no wound but does mean urinating is very painful. They are also under GA for a shorter time and when DS woke up he was very distressed. The pain in urinating lasted for around a week and was very difficult to manage as he was holding on till he basically wet himself.
That operation didn't work however, made absolutely no difference to him, so he had to go back 6 months later for a full circ.
This was actually better - the wound needed a lot of care but there was no pain with weeing so it was all much easier to handle. And he was under GA for a lot longer so woke up more 'naturally' which was much easier for him. He had to stay home for a couple of weeks (or rather, not be involved in any rough and tumble play, which for us meant stay home 
) have baths twice a day and use vaseline in his pants to stop sticking. The only really tough time for Op 2 was when he woke up the next day after sleeping 12 hours and didn't have any meds on board - it was a tough couple of hours till they kicked in.
So that's the two ops that he's likely to have. If there's a choice, in terms of the ops I think the full circ was much easier to cope with than the partial. PM me if you want to know any more, sorry this was so long, and btw a medical circumcision age 5 is not an amputation (but you knew that anyway).