most guys are liars, and i know that this is a major generalisation. however, i stand by this claim. esp. around this age, guys dont know wat they want; perhaps its a gf, they might want to be single, they want to cheat... watever...
the main point is, this guy just did the same thing most confused and disorientated boys do --- thought he wanted something (U) , maybe forced himself into thinking that he's over the ex and he has to move on, then perhaps the ex gf made a move and he realised he just wants to be with her again.
dont take it personally, im sure he meant all that hooking up stuff etc. but sounds as though he wasnt over the ex. (bitching about the ex in a hostile manner is a dead give away... most of the times)