That sounds scary, I'm glad he's on the road to recovery now!
IME, the subject tends to come up naturally - dd2 had a floppy larynx and was very ill as a baby, then had larynx, tonsil and adenoid surgery aged just 2. She doens't remember any of it, but does love stories about when she was a baby, and asks questions - I just make sure I include little age appropriate snippets of truth. Illness is just a part of life, and has been a very small part (to her) of hers, and I just make it matter of fact. Kids manage facts very well.
The main challenge, for me, was being able to separate out my own feelings and memories before talking to her about any of it - I was very scared at the time, and I didn't want that to impact on her. I did lots and lots of talking about it, not all the time but when it came up, for at least a year after she nearly died - and it got easier.
Does your ds need to know specific things, in order to make sense of medication/limitations etc now? Honestly, just keep it truthful and matter of fact, and age appropriate language - he'll be fine. If you've been supported/are supported around his illness, he's very unlikely to be scared - at his age, he mostly needs to know his parents are ok and there for him 
Hope his recovery is fast and happy.