Hello, my name is XXXXX XXXXX it is my pleasure to assist you with your question today.
There is no specific document which proves the legal owner of a car. The registered keeper is not necessarily the legal owner and the two are entirely different. The legal owner is either the person who has purchased the car, or a person to whom the buyer has gifted the car. So a receipt of purchase or proof of gift are the two most common ways to prove ownership if it comes under dispute
yes that can suffice, for example something in writing that confirms you have gifted the car to him. There is no formal document to confirm a gift though, many gifts are jst given with no accompanying documents and it is the intentions of the parties that transfer the ownership
That is a good start and make no promises or statements that you are gifting him the car
But in the event of a dispute where the only evidence is a receipt in your name, it would help you more than him
you do not need to prove you are the owner of a car to sell it, you can do this just with the logbook if necessary
But then in the event of a dispute the legal owner can challenge the sale and try to overturn it
yes that is possible unfortunately