Hi Michelle.
Welcome to the site 
Accommodation in Bordeaux is unfortunately expensive and relatively sparse compared to other cities. You may want to make sure you’re sat down before you look at rental costs here.
Probably the best place to look is a site called Le Bon Coin (www.leboncoin.fr). It’s a bit like Gumtree in the UK, and Craig’s List in the US. As well as being somewhere to buy & sell all sorts of random stuff, it has a large “immobilier” section, including house and apartment rentals.
However, I seem to remember having difficulty registering an account on Le Bon Coin whilst still living in the UK because it required a French mobile phone number to receive a two-factor authentication code. Hopefully that’s no longer the case.
Two popular sites that are the equivalent to Right Move in the UK are Se Loger (www.seloger.com) and Bien Ici (www.bien-ici.fr). Both cover rentals but I’m not sure these are really tailored for student accommodation.
Another one that springs to mind is a site called Particulier-à-Particulier (www.pap.fr), which unlike Se Loger and Bien Ici is for direct rental via the landlord rather than via an agency. However you may find not all landlords speak English and dare I say the risk of stumbling across a dishonest landlord who’ll take advantage of you not knowing the French laws is higher than if you use an agency.
Are you able to visit the city easily to try the local letting agencies?
I’d recommend him posting to the Bordeaux sub-Reddit too, as there’s often students posting there looking for accommodation.
Bordeaux is an amazing city and I’m sure your son will have a blast once he’s here.