Nope, unconstitutional. States don’t get to add their own requirements.
What he’s focusing upon is the primary only, and believing that the state has some control over this. In actuality, the Republican Party is the only one with some control or standing over who is listed in the primary for their party.
If they’d written this and said that ALL candidates for ALL offices, have to provide some tax return...he might have gotten the interest of the Supreme Court to hear the case and be entertained by it.