can i connect a DJI air unit to my pixhawk 2.1
Depends on what firmware are you using. As long as ardupilot and px4 supports, it should work.
This system is not directly compatible with Mavlink. However, there is an MSP to MAVLink Arduino converter maintained by D3NGIT that allows it to provide telemetry data via the custom OSD.
What air unit it ?
The Ocusync Air system supports Mavlink however the new DJI Digital FPV system does not Suport Mavlink and only works with MSP.
Ardupilot team and Alex have been working on adding MSP for OSD into Ardupilot and I directly supported them with early builds testing the firmware and lending the my DJI FPV system. I believe Master has this support in now but you would need to check with them. There is a dedicated thread on this on the Ardupilot forum.