Your dna does not change no matter what you eat.

Absolutely right, your dna is fixed but there could be a solution. If you are absolutely sure there is no other person involved, have both of you checked for chimerism. This is a rare condition where you can have different dna in different parts of your body as one (or both) of you were conceived as a twin but absorbed the twin early during development. Might be worth bringing up. But as said before, eating does not change your dna no matter what you have eaten.

If the dna sample was taken by using a swab from the mouth, were you present when the 'father' did his? Could he have possibly got someone else to do it in order for the result to show he is not the father? Just an idea!