The UK has not left yet, so he has done virtually nothing as of yet. Saying that his political ambition has been achieved is just an empty excuse.
He could
- see it through!
- be the valiant defender/representative of the Leave voters, now that everyone has abandoned ship
- bash the UK parliament for not voting on the matter and legitimazing the referendum
- urge Cameron to invoke Art. 50 asap
- attract voters and campaign to be the Brexit prime minister
- do something worthwhile against "backsliding"