“ Except the states could prevent him from accessing their ballots.”
No, States can not add any additional requirements to what is in the U.S. Constitution. If he is nominated by the Republicans, the States must place him on the Ballot.
Mighty big "if" you've got there, FRiend.
Considering the deafening silence from the GOP hierarchy on this matter, I can see the Republican party keeping him off the state primary ballots. Remember, the GOP NEVER wanted him as nominee OR candidate in 2016.