my dd developed a bad squint overnight. on the friday her eyes were fine, on saturday morning, her left eye had turned in. she was 18 months old. until that point we had no idea she had vision problems. it was 2 months before we got a referral appointment for her to see an opthalmic optician at the local hospital. obviously, we didn't have the added anxiety of the bumps on the head your ds has had, but to answer your questions, yes, squints can appear suddenly, and yes, you can think a child has really good eyesight when in fact, they don't.
dd's left eye is pointing at her nose. from my understanding of squints, it's usually because one eye has poorer vision than the other. for a while, the brain tries hard to use both eyes, then one day, the brain recognises that one eye is weaker, so stops using it. the muscles in the weaker, now unused, eye relax, and it drifts inwards. it will stay there, until glasses are introduced. the lenses of the glasses (most likely a different prescription for each eye) compensate for the poor vision, making both eyes equal. the brain then realises it can see through both eyes, and begins to use the weaker eye again, muscles are flexed in it and the eye is pulled straight. sometimes this doesn't happen, and needs a little kick start, which is when patching the good eye happens, which forces the brain to exercise the muscles in the weaker eye.
dd was picking up crumbs from a patterned carpet a few days before the squint came, so it was a real shock. she has very strong lenses for both eyes.
from one of your posts - "they are going to try and treat his vision rather than correct the squint (he said it would be years before they would think about correcting it as they would try glasses/patches when he's older first),but i think this will just make his normally good vision deteriorate and think it is pointless treating his vision when it has never been a problem before" - the issue here is children's eyes are changing all the time until roughly the age of 14, and sometimes before that age, squints can correct naturally. if they operate too early to correct the squint, and pull the eye straight, when/if the eye corrects itself, you will end up with the eye pointing out as it's now been overcorrected iyswim.
sorry for the long post, i'm not a medical expert, but have learnt a lot about childrens vision over the years, both dc's have squints. dd's vision is by far the worst, we've done patching for 9 months too