I had a similar situation, where my DO changed offices, and didn't seem to excited to write me a letter. (In fact, when I asked him, he laughed and said "Pre-meds are always trying to get me to write letters for them." Not good.)
So I took 2 boxes of Krispy Kremes to his office. Gave them to the staff, told them that I wanted to say thank you for the DO writing my LOR. I attached a thank you letter, a sheet with all of the addresses of the schools I was applying to, and a bunch of stamped, already-addressed envelopes.
It worked for me! Good luck!