It's not a Newcomb problem. It's a problem of how much his promises mean.

Either he created a large enough cost to leaving if he is unhappy, in that he would have to break his promise, to justify his belief that he won't leave; or, he did not. If he did, he doesn't have the option to "take both" and get the utility from both because that would incur the cost. (Breaking his promise would have negative utility to him in and of itself.) It sounds like that's what ended up happening. If he did not, he doesn't have the option to propose sincerely, since he knows it's not true that he will surely not leave.