When he was within his term of office then as POTUS he could reveal classified information to anybody if he felt it was appropriate. Previous presidents have done precisely this, disclosing classified information during state of the union addresses, etc. The important thing to keep in mind here is that the material actually remains classified. Just because the POTUS mentioned something classified it doesn’t mean everybody in the White House, military, CIA, etc. are now free to talk about it as well.

At the same time he could declassify specific material, typically via an executive order. The key is that this is a formal process with a paper trail that lets all the appropriate governmental agencies, departments, etc know precisely what is being declassified. He can’t just verbally say something like “this box of papers is now declassified”. At the very least there would need to be a printed list of exactly what is in the box.

The minute Trump left office he lost the ability to perform both these things. At that point he’s basically considered a civilian with a security clearance. He can have access to appropriate classified material, but he’s not at liberty to disclose any of it.