Before actually submitting anything, I contacted one of my ex-supervisors and he agreed to write a reference letter for me. That was almost a month ago. Then, the first time he received the invitation to submit a reference for me (invitation self-generated from the online application system), he promised he will submit it "soon" with enthusiasm.
However, days went by, after two weeks of the initial invitation, nothing was received. I therefore sent a reminder to him and he again said he will submit it in one to two days (he did mention he was a bit busy). Now a week has passed and still nothing. The deadline for getting the reference is closing soon (in two weeks).
What should I do? I have options to send him an email reminder personally (I cannot call him because we live in different countries right now). Or I can re-send an invitation via the application system. Which one would be more polite, as well as more effective in getting him to actually write it?