It is hard but one of you could be wrong. Here is why:

1) your story,
2) His story, He can say while he was driving but suddenly something hit his car and he noticed that you were changing the lane but driving too fast and hit his car in rear left end. This could happen if there is another lane on your left at that time. If this left lane is there you might have little bit chance to win.
3)If you were in the left-most lane, i.e. there is no more lanes in your left. Then, you are guaranteed to win the case.

Definitely,this guy is a very experienced driver with good knowledge to handle accident. He knows all the tricks and he is going to lie.

Good luck.