Hi,
There are two scenarios.
1. Either he gets promoted without completing his degree
2. you wait for him to complete his degree to promote him.
I would ask these questions.
1. Why do you want to promote him? Is it because you have a role for him to perform, or he is getting promoted just like that?
If the first case, dont wait for him to complete the degree. If otherwise, i assume, that your organization is not going to lose much by not promoting him, then insist on completing the degree.
2. Can your organization afford having this person quit (worst scenario). If yes, again insist on completing his degree?
Also evaluate what is that you gain out of him completing his degree as an organization. I would suggest that you promote him and link his compensation revision based on his completion of degree. Commit to him, what is he required to do and set that as a target under his KRAs.
Hope this helps.
Regards
Vinitha