Yes, he can be tried.
I get that most people here have never heard of Belknap but by now that name should be getting a lot of play.
That was not a President.
Belknap is not much in the way of precedent and certainly is not a controlling one.
William Belknap was the Secretary of War under President Grant. He was impeached by the House, resigned, and was then tried by the Senate. He argued that the Senate no longer had jurisdiction since he had resigned, and a majority of the Senate voted that they did. He was then acquitted by the Senate and the issue became moot.