Hi! I'm new here! But your post caught my eye. Seems your boy started doing this WAY before my boy did!! I would first try to figure out HOW he opening the fridge, then try preventive tactics from there.
My Chin is about to turn 9 years old and I swear his intelligence ranks higher than most humans. Chin too likes to open fridges, as well as cabinets, peanut butter & catnip jars and even some household doors.
I have tried everything to persuade him to stop. The only good(?) news is that Chin seems to go thru phases where he is obssessed with the fridge; then others when he could care less about it.
Since Chin is an observant and fiesty fellow - he realizes he is not quite tall enough to grasp the handle, so he lies on his back and pulls at eh rubber seal around the fridge door until he opens the fridge, goes in and retrieves his hearts desire in food. (This is also the same way he opens cabinets and doors that dont quite latch) The only thing that ever worked and he could not somehow "move" out of the way was a 75 pund toolbox, closed and locked in front of the fridge door, near to where he grabbed at the seal. Since he is a very large boy (I'm convinced he's part Maine Coon - 22lbs and stands taller & longer than most small dogs) putting chairs, full paint cans & other things didn't work as he would just shove them out of his way or crafity work his way around them.
Chin can also open peanut butter jars & catnip jars by holding them in his fron two paws & kicking withhis back feet to "spin" the tops off. And he is so annoyed he doesn't have the opposoble thumbs to turn doorknobs! Though that doesn't mean he ever quits trying he knows HOW they work he just can't get them to cooperate! We had to replace all the door knobs with handles that you just push to open, because he figured them out.
He's a handful - though I wouldn't trade him for anything in the world!