My suggestion would be to drop the issue and let him come to his own conclusions. If the stuff is a problem help with organizing. If there is an out of the way place to hide unopened boxes they are not really a problem.
You don't want your help to be perceived as a personal "attack" of his stuff. This could create a problem with your relationship.
Going minimalist is great. I moved and have done it so well. I love it! But you have to want it.