Again, why would you need to have him sign them if it isn't forming part of an application to any authority?
If you own a plane and build hours in it, you fill up your logbook and sign it to say you've been honest.
If you want him to sign it, I'm sure he will, but I really don't see what it adds. You should sign your own logbook to legally state that the hours are true, if you then get audited and they find they are not, it's you that's in trouble not this other guy.