Brego is not even 4 months old. Hard to know what goes on in their puppy brains, so this is only a wild guess. I do appreciate they are incredibly sensitive and reflective to our moods. However, I certainly wouldn't try to understand what is going on in his mind from a human perspectives like jealousy, or protecting, or understanding the direction your relationship should go.
He is a very young boxer pup, and as such always looking to play and get attention. He may simply see your bf as someone closer to his status, (given your fighting), and someone to engage and play with. Very common for boxers to get engagement by doing things people will respond to. Stealing clothes from someone so that person will give chase or respond in some fashion would not be surprising. Meanwhile you may be less approachable for Brego play if you are irritated with bf.
Just some wild guessing.
Good luck with your pup and bf.
Mom and dad to Layla b: June 2, 2008 and Jack b: Aug 19, 2010
Flashy brindle girl and boy, both docked and floppy,
Forever in our hearts: Sally, Maxine, and Lucy - all at the bridge.