He's living in a fool's paradise about his chances.
fool's paradise
idiomA state of happiness due to illusion or false hope.
Origin
first recorded 15th c.From Middle English foles paradise, folis paradyce, first recorded in the Paston Letters (1462).
She was in a fool's paradise, believing the deal would close without any problems.
The investors dwelt in a fool's paradise, ignoring the warning signs until the market collapsed around them.