Many thanks again to everyone who offered advice or encouragement over the issue of year abroad accommodation for my son.
As of yesterday evening, it looks like we've got a result. He finally received an email from the university accommodation people confirming that he's got a place and he's got instructions on where to report and when to collect his keys. (He can collect his keys from 1st September, so he heard with a whole week to spare.) With hindsight, it's just as well we didn't book anything else for him.
As far as I can tell, this is normal. He applied (using a paper form which was handed in at his home university and then sent to the French university by a deadline in by mid-June. Not hearing anything was bad enough, but what made it worse is that some others from his course heard about their accommodation a month ago, but as far as we can tell, that's because they bucked the system. In addition to filling in their forms, they also applied online, and it was the online application that got them the early reply. So, in all probablility, they've actually got two places each. I think the logic is that most people need to know in advance whether or not they've got somewhere to stay, but ERASMUS students are 'guaranteed' a place, so there's no need to tell them anything until the last minute. And even then, they don't get any information unless they email several times. It also seems to be fairly normal for students to turn up at the accommodation office having heard nothing at all. All I know is that it makes the whole process much more stressful than it needs to be!
Thanks, everyone, once again.