I am not a parent or a medical or sleep professional, but a person who has sleep apnea who has tried to find out what I can about this condition. I also have experience around kids with autistic spectrum disorders, including asperger's syndrome, which as you probably know has some similarities to HF autism and ADHD. I understand that mood disorders and gait differences can also be a part of these disorders.
I wanted to let you know that kids your son's age can have obstructive sleep apnea, and/or central SA, hypopneas (diminished breathing flow), and/or upper airway resistance. Until your son has the sleep study you won't know what mix of these he has. His symptoms may have started with the medication, or may have worsened and become apparent since then. It's probably impossible to tell if someone is having hypopneas or airway resistance from looking and listening, especially without being a health professional. The measurements they take in the sleep study will show what is happening when he sleeps - and if he needs various types of measuring on different nights he should get that - there are ways to do it at home. And if the medication caused his difficulties or worsened them, we don't know whether it did any permanent harm... often things improve when the medication is stopped. Treating the apnea could well reduce the need for the meds too.
I think the brochure you read in the hospital isn't very relevant here... although I know you were just trying to find out about apnea to help your boy. There is a huge push to make adults aware of the long term risks of untreated apnea, because many people think it's not so bad and it doesn't need active treatment. So the mention of the effects on the heart and brain are part of those scare tactics aimed at a different audience, who often already have those effects underway. The reference to SIDS was probably just to try and explain the concept of an apnea to people who know little about it... for many members of the public I guess SIDS is the only similar thing they are aware of. But it is very, very different. The cause of SIDS in many cases remains unknown. Because your son will have a sleep study soon and get treatment, any long term effects he might have otherwise suffered, should be prevented with treatment - which is great news.
One clue that there may be obstructive apneas involved is that he improved on antibiotics. This could have been reducing the size of his tonsils and nearby structures probably inflamed by chronic infection. The red appearance is of course another sign of infection.
In hindsight, it would have been better if the doctor had kept him in hospital for 2 nights, because breathing problems are a known risk following anaesthesia, especially in suspected sleep apnea, and so is inflammation and swelling of the airway caused by the actual surgery. There are some handouts on this website for future reference for all of us - people with any sleep breathing disorder should not have day surgery/outpatient surgery. We should be kept in overnight at least, and observed for any decline in breathing. Even being left alone in the ward is a risk, so having monitoring with an alarm is advisable. We also usually feel better with oxygen on overnight - I know I do, it keeps my oxygen saturation up at 97%, my awake level.
You probably already know that his sensitivity to food textures is probably related to the HF autism. You are on the right track with the reflux - it can be silent, it can also irritate the adenoids... its even been known to get into the inner ear in some cases. I also forget to breathe sometimes when awake, some of us here do... i also get looked at like i am crazy when i say that :)
Good on you for getting him tested... it's totally understandable how scary it is at first... as you get more information from the studies and doctors, hopefully you will be able to not worry as much... knowledge is power.
"Attitude is a little word that makes a big difference" - Winston S Churchill
Machine: Respironics M Series Pro, 17cm
Mask 1: Respironics Comfortgel nasal
Mask 2: Resmed Quattro full face
37yo female, New Zealand, Mixed SA, AHI 45