Sorry to hear of your lad's injuries and his passenger's...
As for the crash, again
generally speaking you could blame the road only in very very extreme circumstances, in all other events the driver is expected to anticipate (sometimes 'guess', if you like...) the road condition/direction and if unsure, take precaution e.g. slow down and confirm....
A solicitor once told me that if you are involved in a crash, never say 'I did not see him' (the pedestrian, cyclist, or other vehicle, as the case may be) because the answer will always be 'well, you
should have...'. What you should (try) and say is 'I
did see him, but was unable to avoid the crash because of a, b, and c,' (e.g. he behaved erratically and in a sudden and unexpected way).
As for this one, sadly there are enough clues in this junction for a competent driver to know what he is driving into. These are the post signs and the white line running perpendicular to you son's direction of travel. These in my opinion are enough for you not to be able to claim extreme road circumstances, plus it was during daylight or early dusk at most so sorry but no excuses really....
I appreciate that he is a probably a responsible and careful driver and no boy racer, but even experienced drivers get caught-out sometimes (I know, I did, more than once...). In most cases and with a bit of luck this will end up in a 'phew, what was that? who put a junction here? good job there was no oncoming traffic', but sometimes it end-up as it did here. Still, could have been much worst... 3 broken ribs and a few penalty points is still quite lucky I think. Hope he gets well soon.