Transferring a private registration

[Blondee666UK]

Hi, can someone please help me. I have given my old car to my son. The car has my private registration on it. I was going to put it on retention, but I'm worried he wouldn't be able to tax and insure immediately. Can he tax and insure if the number has been put on retention? or do I need to wait until his new V5 comes back and then he transfers back to me? Many thanks