Dad and 21 year old  son make a contract

Dad and 21 year old  son make a contract. Son is a smoker and Dad is disturbed by this fact. Dad promises to Son if he can quit smoking for one year, Dad will pay Son $5,000. Son quits smoking and does not smoke for whole year. When the Son  inquires  about the  $5,000, Dad says "Son, I got you to quit smoking, this is worth much more that $5,000, therefore, I won't pay you that $5,000." Could son take Dad to court and sue for the contractual obligation? What issues relating to the consideration of the subject matter of the contract are raised here?   4a) Would the outcome of the case be any different if the cigarettes were of marijuana variety?