He isn't trying to get to the room, he's trying to get to Daddy. Does your husband work from home or otherwise have to spend time in that room during your son's waking hours? If he does, how does he know he's there? Is there a glass door that he can see him through? If that's the case, put a curtain or shade there so that your son can't see in. If he doesn't *have* to spend time there (for something like work) then have your husband hang out in a more accessible part of the house that's already baby-proofed until your son goes to bed. This is just a temporary stage. If it's a solid door and he can't see him, try making a "goodbye" routine for when daddy has to go to work in the office, just like you would if he were leaving the house for work and then make sure your husband stays in the room and isn't in and out every few minutes all day, disrupting things.
When my kids were younger, they had a hard time staying out of my work space when I worked from home and had a sitter so I would have to be very disciplined about actually working in my work space and only coming out for lunch and bathroom breaks, just like I was in the office.
I'm a little curious about what mattresses have to do with this...are you saying you had to child proof your bedroom as well? Why?