I promise my son that if he goes to Europe

I promise my son that if he goes to Europe for six months to see all of the sights and gain some worldly experiences, I will reimburse him for all of his expenses when he returns. He takes the trip and then comes back demanding the reimbursement of his expenses. I refuse. He sues me for breach of contract.

Can he prevail? If so, under what contract theory will he prevail? Be sure to define and provide the required elements of the selected theory. What type of defense, could I raise, if any.

300 words.