Some places will ask for a 'holding deposit' whilst students decide if they want the house. It's v dodgy, preying on students' housing panic and the advice from my uni was not to pay it (same at my sister's uni). OTOH DSis paid her holding deposit for this year's house as she wasn't able to get back to the town to do a search so just wanted it sorted. As your son is on-site I would say not to pay one, if asked, and keep looking - I didn't find my student house until March and moved in during July, although some had theirs before Christmas (and got stung with the hefty 'holding fees').
The other piece of advice is to check the deposit, when you do pay, goes into a government approved holding scheme. This means if damage happens or the LL claims it has happened the govt looks after the money whilst a mediation process takes place - stops the students from running away and the LLs from making unreasonable claims.
Re: moving in September. He may find that a previous student wants to sublet the room over the summer, or find another (usually international) student willing to sublet. This would have to be cleared by the LL, but I've never known of any problems with that (maybe I've just had nice LLs!) :)
Does your house insurance cover him or will he need his own? Lots of student houses have poor windows etc. - I know of several laptop thefts from downstairs bedrooms and bikes from yards. Uni should have free UV marker pens.
If there's a housing list with private landlords I would use that over agencies - DSis is currently using an agency and they are terrible (if your DS is in Bath PM me and I'll tell you which one so he can avoid them, I wouldn't wish them on my worst enemy!) and they also charge extra fees. Check that the LL is registered with the uni and most uni housing offices will also check the lease for students so they don't get scammed.
If I'm sounding anti-LL I'm really not - I've been very blessed to have some great ones (including a lovely woman who wanted to put underfloor heating in our kitchen... builder talked her [quite rightly!] out of it
). OTOH I have heard of some stinkers.