Re: His Personality Changed.
I think they have all hit the nail on the head. Whether it is a fear of losing you or jealousy of other guys looking at you, it all leads to one thing. He is obviously not going off you, I'd say that the point is he has become more attached to you and is (maybe?) a bit possessive of you.
Sit him down. Say that you have noticed a change. Be understanding and allow him the freedom to get whatever it is off his chest.
~ The journey of a thousand miles begins with a single step ~
* Nothing in the world has ever been accomplished without passion *