Most people who keep their phones locked and avoid sharing their password to their loved ones have something to hide. This is a behavior which is found especially with cheating spouses. If he does not want to share his password, just let him be, as long as he is faithful to you.
I keep my phone password protected and locked. I don't have anything to hide from anybody except from potential thieves. If my phone was lost or stolen I wouldn't want anyone having access to all of my info. Only a fool wouldn't lock their smart phone these days. Exchange passwords when you're both comfortable.