How to create a Linux user without a password but being able to set it?
You have his SSH public key? Put it in .ssh/authorized_keys in his home directory. Once you have done that (and the permissions are restrictive enough - SSH is fussy about that.) he will be able to log in without needing a password at all... and he will never need one.
The best part is that his public key is not sensitive, so passing it around in emails or chat is fine, and his private key never leaves his computer.