I agree - questioning is essential - my 20mo niece started to get ill one November, my sis took her to doc, went to hospital, she had sickness and loss of balance so they diagnosed tonsilitis (WTF? Her throat wasn't sore, her tonsils weren't inflamed) OR teething probs.
She didn't get much better. She got worse, balance-wise, and she had a couple of fits. My sis told the GP who didn't really believe in the fits but said they were probably febrile convulsions (neglecting to mention that a fever should be present for febrile convulsions)
By Christmas Day she couldn't walk (no balance, feet flexed semi-permanently) and was sick but then could eat straight after. And she had a fit while doing Row row row your boat. I was convinced at this point that she had something neurological, but thought maybe it was a blood clot (she had smacked her head so hard so many times from falling over due to loss of balance ) I tried to persuande my sis to take her to A&E on Boxing Day but she wasn't having any of it.
Eventually, a couple of weeks later (and after a few futile trips to the GP) she was taken to a Paed A&E unit (UCH) where she was diagnosed with a brain tumour that was causing massive intra-cerebral pressure - much longer undiagnosed and she could have had real problems, possibly even died - as it was, they kept her in and put a drain (shunt) in the next day - the pressure release shot fluid all the way across theatre, apparently. 2 days later she had the golfball sized tumour removed and she is now ok.
Sometimes HCPs really don't know best and you have to keep asking/pushing to find the truth.
Her consultant neurosurgeon said that it wasn't surprising that the local hospital/GP hadn't recognised the symptoms (WHY? Even I could guess it was neurological!) because it is unusual in one so young and they aren't likely to see many cases - he even told us about a neurosurgeon whose 14-yo son had a brain tumour and it took ages for him to realise, he just thought the boy was trying to skive from school with "fake headaches" .
And I second what Technofairy said ( for you TF - glad your DS made it through) - watch out for loss of balance, light sensitivity, waking up with a headache every day - my niece cried every time she woke up but couldn't articulate why, and she couldn't tolerate anything on her head, hats, hairbands, nothing. Push and PUSH - it could be an inner ear infection but it might not be - they tend to only last a few days anyway, so anything over a week, get pushing.