Hi Fussy, My DD went to Germany (Hanover) as a student through Erasmus scheme - getting accommodation was rather daunting as uni let her down very late so as I'd gone out for a w/e we had to just focus on finding her a room. We looked in a student book which said look at notice boards. So with help from restaurant staff we tracked down boards - then by chance a woman saw us there looking a bit desperate and asked if she might be able to help as she and a group of friends were buying a house and planned to let out a room!! It seemed a long shot as they didn't actually own the property at that pioint - but we went and had a look and DD moved there about a week later. It worked out well - amazing really.
However if that hadn't worked we also found an International Student Office which had an Accommodation Officer and I guess that should normally be the starting point.
Has your DS got any framework such as Erasmus or an Assistant job? Withough those yes It might be difficult to know where to start but their uni's should really make sone suggestions eg putting them in touch with students in the same town the year before.
I did 2 YAs (many years ago)in France but was given accommodation each time . A niece did an Assistante job in France recently and was again offered a room. Another niece is in Bologna and somehow found a place. So fear not, it can be done! I think students use FB a huge amount to find people in in similar situations and get tips. I also think they have to be quite flexible about what sort of accommodation they will accept.
If he's going to a uni town the website will give some pointers and he might find a student guide to the town, for foreign students, as we did. (We found it by chance as someone had left it lying around at the Youth Hostel!)
Good luck - I'm sure he'll be fine. Toulouse is a great city to go to by the way! Where is he thinking of?