Ok, well then, if I were you, I'd want to find out what 'being bipolar' means to him. It may well not have the same associations for him as for you... Is he using the term casually just to say his moods go up and down, or does he mean he thinks there's something wrong with him? Does he think it's a bad thing, or does it seem exciting? Is he distressed, upset and looking for help? Or is he looking for an explanation for what he's feeling? Or even, is he boasting? There are a lot of famous bipolar people, like Russell Brand, Macy Gray and Stephen Fry - so some teens may see it as desirable to be bipolar.
As his mum, you probably have two main reactions: you want to understand more, and you want to know what to do. If you're like me, you automatically worry, too! The tricky thing is you can't work out what to do, if anything, until you understand more... So you have to start by talking to him.
How you talk to him about it probably depends on how you found out. I always think direct is best when possible, so I'd probably just ask "School tell me you have said you think you're bipolar. Can we talk about that please?" If he told you directly, then I'd take that as an invitation to talk already, and I'd suggest something like "I've been thinking about you telling me you're bipolar... I'd like to understand more about what you're feeling and thinking"...
Then, if you find out he's serious, you can take him to the GP... If he's worried, you can reassure him... If you find he was being flippant, you can relax, but maybe take the opportunity to talk to him about mental health generally...
I'd also add that he's been through a lot, and it's quite likely his emotions are up and down and all over the place. He may need reassurance that that's normal, and not a sign of mental illness. Feeling down and depressed is to be expected, under the circumstances... But feeling madly happy is also normal for teens! When my mum died (I was 19) I can remember feeling guilty every time I laughed or enjoyed myself, and worrying that I might be going mad because I could feel miserable and happy in the same day. Your DS might be feeling something similar...
Good luck with it. :)