Thanks everyone for the advice. I think that I've decided to keep bothering him about once a week or every other week and hope that he surfaces.. worst case scenario he can write a late letter that will be added to my record. Like I said, he and I are very good friends, so I have his cell phone number and I've left him 2 messages. I have a feeling that he is in India (his home country) and I don't know if he has his cell phone/is checking his email. I may ask another professor in the department to confirm for me and to see if he left any alternate means of communication.
I definitely agree with pre_JD_2be's advice re: setting up the LSAC account early. Since I'm applying to PhD programs too, I decided to wait and ask for all of the letters at once, rather than feeling like I was nickel-and-diming them every time I needed a letter.
In any case, I am going to have my boss write a letter and I'm going to scrounge up a letter from another prof, just in case. It won't be as strong, but I think that it is better than having only one academic letter.
Finally, how late is too late to send a letter?