Your boyfriend is only 17 so he is still technically a minor - right? So the state is still responsible for him. Have you or he approached social services? Have you asked them about finding somewhere he can live while he finishes school or looks for work? He isn't an adult yet so the state can't simply abandon him can they? I don't know exactly where you are, and being British doesn't help, but have you looked online for any charities that might be able to support him in his area? Have you run through Google for things like "help the homeless", "shelters", "child shelter", or things like that with the area code or name? I know it's only a first step but I can't believe there aren't organisations out there with a presence on the web to help you find them and who would be able to help him get back on his feet.
Right now that should be your first priority - help him to find a roof over his head and a way to feed, wash and keep warm at night.
I guess you must be very scared right now. You must be petrified of what's going to happen when his cell runs out or whatever. I know you don't want him to be alone. Make sure that he has your contact details NOW. Make sure he has them somewhere safe, that he makes several copies of them, and that he reads them back to you. It sounds highly likely that a point is going to come where you will be out of touch and it's very important that he knows how to get hold of you again afterwards.
If he is homeless is there no way that he could make his way up to you so that you can at least be together? I think it would really help him to have your friendship and support right now and if he could somehow get up to where you are then you could have contact with him and you could look for local organisations to help him together.
SO your first priority should be those life-sustaining practicalities, but your second, I think, should be to try and get him up to you. Having someone who cares and having a second mind working on the problem would make such a difference and you are more likely to be able to sort something out together than on your own.
Apart from that I really don't know what to say. How did this whole situation come about? Has he been to charities or to social services before? Why have they not helped him - being a minor and everything you'd think they would?
Please feel free to submit another request or PM me with more info. I'm sure there is some way you can help him, but if nothing else make sure you get him those contact details like i said and make sure you know his exact last known location.
I really do wish you the best of luck.