
Definately go get some advice, the CAB are brilliant they really know their stuff.
He *isnt* technically homeless if he is living with you - unless you go with him to your local authorities housing department and state that you are not prepared to have him in your home... which since you are willing to house him I wouldnt recommend you did. (If you did then he would be high priority for immediate housing... however that would most likely be a hostel (best case scenario a specific young persons hostel with support workers who help kids out with work/college/lifeskills/moving into their own flat - worst case scenario... unsupported housing such as B+B's and bail hostels used by those just out of prison! - having been there myself if you can live with him, do try to do so!).
However, if hes in full time education and you are taking the legal role as his 'parent' or guardian then you will be due benefits towards his care - the CAB will advise you on what you are entitled to and what you need to do to get it. What I do know is.... act FAST these things take an age to get moving unfortunately and can only be backdated by a short while (max three months for some things, less than this for others).
You'll really need his parents to state that they are passing responsibility for him over to, that he is no longer living with them (especially if they are liable to try continue claiming whatever benefits one currently gets for having a child in full time education!!!).
Go and speak to his college and they will let you know of any grants/help available, especially wrt things like laptops/computers etc. However most colleges have very good computer and internet access, and if they dont, libraries do (even my tiny 1 room community centre based library provides five computers that can be booked in advance or used if they happen to be free at the time, the whole place is barely big enough for a set of shelves let alone a libraries worth of books!! :D)
Getting the details sorted out asap will take a lot of pressure off him - they can easily be changed back should he go back to his parents at any point (I have no idea if thats an option but dont let the possibility of it put you off or delay you in getting things in motion).
Colleges are not really geared up for kids who live independantly of their parents so this is not a route id recommend - its VERY hard negotiating the change in education style for a start without being totally adrift setting up your own home or surviving the unfortunate downsides of life in a hostel. The nail in the coffin for me wrt college was the constant going on and on about parental permission for this and that and having them sign things for various exam fees or grants etc, especially combined with other kids finding out I had my own place and thus was 'cool' and would undoubtedly allow them to hang out at mine and party all night long (nuh uh!!!!! but a LOT of people in their first 'own place' do get sucked in and it really is incredibly hard to discover your real friends from the users and hangers on at a time like this).