Hi Emma,
My DS does this also, especially if he is tired - and my mum lives just over an hour away. It used to distress me at first but now I don't worry about it. I know he's OK, just tired. I never stop as I prefer to just get to my destination rather than prolong things by trying to settle him (I don't think it would work anyway). The longest he has screamed for is about half an hour or so. I turn the music up (not too loud) and tune out as best I can - singing helps. I think that they grow out of it and will get used to it eventually. Crying isn't the end of the world, IMO.
Good Luck.