Sending files back and forth question

A friend of mine and I are recording an album together.  He's in Florida, I'm in RI.  He's using Sonar X2, I'm using Sonar Platinum.  I recorded mostly all the instruments of the song.... basically a backing track.  I did record to a click, which means each song has a specific tempo.  I sent him mp3's of the songs, he imported them and sang the lead vocals to them.  Then, he exported them as WAV files and sent that back to me, which I imported back into my project.  Should line up perfectly fine right?  Well, it didn't, unfortunately.  There was a noticeable difference in where his vocals were in relation to the music.  
 
Any ideas on what the problem is?  Does he have to set the tempo in his project to the tempo of my project?   

What he said
 
FWIW - I like to use a sync tone on beat one two measures before the start of a tune.  All collaborators use this tone at the beginning of their tracks, providing a full length track from this point regardless of where their part(s) come in.
 
Files are obviously larger, but lining things up is never a problem.  Different folks use different DAWs, so we just exchange mp3 or WAV files via box, where we have shared folders.
 

All of the above and. 
 
- Even though it's not technically necessary for audio, I would have him set his project to the same tempo. For midi it's absolutely necessary.
 
- Make sure when you export that your tracks start at the beginning of the project. Exporting a track that starts at 1:02 will be one beat off if you import at the beginning of a project. You'll need to grab the left edge of the audio in the track and drag it back to the beginning to pad it with silence prior to exporting it.
 
- As also mentioned, midi is tricky as the export will start at the beginning of the midi notes. Place a "silent note" at 1:01 to make an exported midi track start at the beginning of your project.
 
gabo