Senior does not owe Junior the $5,000. The agreement between them lacks certain elements needed to be legally binding:
Consideration: There was no valid exchange or benefit for both parties. Junior's promise to quit smoking was something he should do anyway and didn't provide direct benefit or detriment to Senior.
Statute of Frauds: As the agreement involves an amount exceeding $500, it should have been in writing to be enforceable, but it wasn't.
There's no enforceable contract between them due to the absence of consideration and adherence to the Statute of Frauds. Junior lacks any other legal grounds to claim the $5,000.
Ethically, opinions may vary; some might believe Senior should honor his promise while others argue that since no enforceable contract exists, he isn't ethically bound to pay.