Hi Mad4, have read your post with a mix of sadness and relief - your ds has sounded so similar to my dd2 in so many ways, with any luck he will have the same level of relief from the op.
Having tonsils and adenoids out for severe apnoea is completely different to having them out for recurring infection. When we discussed dd's combo of large tonsils/adenoids plus laryngomalacia with the ENT consultant, he said they would operate from 12 months if needs be.
dd2 was 25 months old, so older than your ds but had the extra complications which necessitated an HDU bed for one night. The hardest part for her pre-op was not having a drink (she couldn't eat much in the way of solids by then anyway, but was desperate for milk). She was asleep when they put her under, so I never went through that, but she did wake up a bit wild and crying which I found hard (she obviously didn't remember it).
Pain levels after vary hugely, lots of others were going through it here on MN at the same time as us - so one experience won't tell you much. dd had a hard time for the first week, and we didn't let up on pain relief - its really important to stay on top of it, giving it a tad before you're 'meant' to so it never fully wears off. Biggest tip: ask for paracetamol and/or voltarol suppositories if ds is refusing to swallow meds. Its not easy with such a young one, and supps. saved our bacon! Our GP gave us a prescription for more when we ran low.
Oh, dd2 ran a temp of 38.5 the second night (first one at home) which freaked me out - we rang the consultant, who said its not unusual for young kids to react with some fever for the first day or two, but to take her in if it stayed up or got worse in case of infection. It was good to know fever can be normal for a short time.
Lots of DVDs, books, small toys, snacks and clean tops for you and him (in case he throws up on waking)...and ice lollies galore (esp. when they won't drink). They advise rough foods when they come round from the op, promotes healing, and true enough dd ate toast and crisps (!) in hospital...but her pain was worse a few days later, when it started to heal, and then it was nothing but ice lollies for a couple of days. She got horribly thin, but everyone kept reassuring me it was temporary and of course, it was!
Best of all, the apnoea was gone instantly - and although she still has a stuffy nose very easily, and snores, its no longer worrying and she rarely wakes up. Loads of luck, will be thinking of you.