When I was in with ds2, I found handcream a godsend, as the handgel you have to use every five minutes really dries your hands out and I ended up with cracked hands before dh brought me some in.
I am so with you on the claustrophobia - I only left ds's room once in a week and that was to go to the nurses' station about three feet from the door.
The things I used most were an A4 pad and pen, to make a note of what the docs said, mind you I was 38.5 weeks pregnant at the time, so had no memory whatsoever (in our case we also used it to record ds's obs, as they wouldn't let me see his chart). Magazines, because I couldn't get into a book for worrying about ds and because of all the constant interruptions and general noise in there - and a book light, so I could read while ds was asleep. In retrospect a small battery powered light would have been a good idea, as the lights seemed to be blazing bright or off, nothing in between, no matter which combination of switches we tried.
I couldn't face the frankly dire food on offer, so dh brought me nice sandwiches from home and my favourite thing was to always have a punnet of lovely big juicy grapes, which were so refreshing as hospitals are always so hot.
I also got dh to bring in some anti-bacterial wipes, after I noticed the cleaners use the same cloth for the sink in his room, table and bedside cupboard as they did for the en-suite shower-room! 
Money for charging up the TV/Phone card, if they have that system at your hospital, as well as your mobile charger. (We were allowed mobiles in side rooms.)
I'd also recommend a cushion or nice pillow as the chairs are anything but comfortable.
Finally, either an Ipod and speakers or earplugs if you want to get some sleep at night. I didn't use either, but have seen them recommended by others lots of time.
My ds1 also has ASD and we thought he was coping well, but had actually just shut down.
First of all it was really tough on him because it was an emergency blue-light admission, so he had no warning, a massive change in routine and of course I was gone for a week for the first time ever. Turns out he was also terrified ds2 was going to die (so were we actually, but we thought we'd hidden it from ds1 well).
He wasn't allowed to visit ,as ds2 was quarantined, but was allowed to come with dh on the last day to bring him home. The relief in his face when he saw his brother playing with a train track was a sight to behold. He has never talked openly about it without us making him, I don't think he can cope with the emotions he feels when he thinks about it. It took him months to get over, but our situation was slightly different, because ds2 was critically ill at the time.
Would a social story about a baby going into hospital and coming home healthy and better help?
Hope it goes well and they manage to get your ds's reflex under control.