I would set as a rule that ANY audio files you guys exchange always start at bar position 1 1 1 1, which should also always be the start of your projects.
You send him your mix as an audio file, he drags and drops it into his Arrange window at 1 1 1 1 and works on his tracks.
If his tracks are software instruments (and you own the same instruments) then he uploads a compressed (.zip) logic file with his software instrument tracks, which you download. You then open your original Logic file, locates his Logic file from your file browser and import the tracks he created.
If his tracks are audio tracks or soft instruments you don't own, ask him to export them as audio files starting at 1 1 1 1 (File > Export All Tracks as Audio Files) and upload those, then drag and drop them in your Arrange area at 1 1 1 1.
Hope that helps?