forgetfullness

This morning I drive down my street and see Jake's bike and helmet in the neighbor's yard. I just shake my head. No surprise. I pull over, get the bike and bring it home. I secretly think of hiding it and to see how long it takes Jake to realize it isn't in the garage.
On numerous occasions we find his skateboard in the front yard or on our front doorstep in the morning when a friend returns it from wherever Jake left it the night before.
After noticing a sweatshirt of Jake's was missing, I repeatedly reminded him to check the lost and found at school and look for it. He kept "forgetting" to check the lost and found so I take it upon myself to do it. I find the missing sweatshirt and two other sweatshirts of his!
His Nintendo DS has been missing for over a month. We've searched every inch of the house and haven't found it yet so I'm afraid this is one thing that's missing for good. How do I get him to stop forgetting and start remembering?!