I mean, it'd be quite difficult to get them the stuff they need in Mexico. You'd have to email them the PDF LOR form, LSAC prefers that LOR's be printed on school letter head so I'm not sure how you'd go about with this, and then he'd have to mail it in from Mexico to PA. Why not ask him via email to write it and indicate it that you plan on applying in September and if it was at all possible for him to have it ready when he comes back in the states, you could meet up with him, give him the LOR form, an addressed and stamped envelope. I'm sure LSAC won't be receiving too many LOR's around that time, and there is hardly going to be any difference between applying Sept. 1st and Sept. 15th or 20th. Some schools don't even put their applications out until the 15th.