We were out shopping and I was going in a different store to get some stuff. He wanted to have a little break so he sat on a bench outside the store. He said he could sit outside with the grocery bag and took the sandpaper out of my hands. Just bought it minutes before.... I come out and we're leaving. He still had the grocery bag. I'm thinking since he stood up with the grocery bag, he probably put the sandpaper inside it, right? Well we leave for the bus and go home. He unpacks the stuff, whatever. I ask him 6 hours later, because I'm done cleaning and doing an assignment, "Where's the sandpaper?" He said he doesn't know. I ask him "What do you mean, you don't know. You packed out the stuff. You had the sandpaper last time I saw it." He said he don't remember. Well I get frustrated. I ask him if he forgot it at the mall. He said no way, because he can't remember he forgot it, he must be blameless. Well who forgot it then? Because it certainly wasn't me. And when I said it had to be him, he gets mad at me and says I am blaming him, as if he didn't do any of this. Why can't he just say sorry? Instead he storms off and gets mad at me for blaming, and "always wanting to be right". Well I don't want to share blame either just so he feels better about himself and his forgetfulness. It's really annoying, and seems like his family also are very forgetful people. I just don't like losing things because he's in his own world (and on the phone)... I just can't deal with this. I feel like this situation is really childish too, but he's too preoccupied with Sidemen on youtube instead of talking about this like adults. I would be fine if he just said sorry and we could move on, but he doesn't want to.

How am I supposed to approach this?? He's not blameless, yet he insists he is because in his reality and memory, he can't remember he did anything wrong.