completly agree with china doll - he may have just changed his mind. horrid for you, but it happens. he may have thought he felt strongly for you - but them seeing his ex, realised that feelings for her were stronger.
unfortunately, when are the dumpee, you will never find a satisfactory answer for why it happened. you will question & analyse all his actions and words - BUT - whats the point now? hes gone - made it quite clear that he's fickle (not that thats a sin, we've all been there !)
pick yourself up and go meet someone who deserves you !
HUGS