As far as I know, all what you need is your ID and his ID with the old ownership card, you do it in the police station (Souq Waqif, or Madinat Khalifa), and it's just two minutes process, and it will cost you 200QR.

I transferred my car ownership to someone else just last week, and I do't know if he did any documents before, but as long as the old owner is ok and present, Why Documents?

Good luck.