There's that 14th amendment thing that could keep him off ballots in states that decide he violated it. A lawsuit on the issue was recently filed in Florida.
All of this seems totally unpredictable.
Florida's not keeping him off the ballot, so I can't imagine whoever the plaintiff is there has standing.
The way it would work is if a state affirmatively decides he's ineligible and keeps him off the ballot (remember, states get to run their own elections), and then Trump sues the state.