Verified answer

No, defamation is illegal.  Telling the truth is not illegal.  A Court usually decides.

They can say anything that is true.

Yes and no.  What you may believe to be untrue is perfectly true to someone else with a different perspective.  Thus even if you could sue for defamation, it can be difficult proving something to be a lie.

There is no law against a former boss disparaging you but most will not do it as it can lead to legal issues. What they will do is confirm employment dates and if you quit or were fired.  But then you come to the real question. "is this person eligible to be rehired"? That usually puts an end to the questions.

I somehow doubt this is the case.

Most employers due to legal issues will not disclose anything more than dates of employment and confirm separation conditions.

If you have proof that he is lying about you then you should get yourself an employment attorney and sue your former boss for slander.