Custody swap every Wed - kid keeps forgetting backpack

So my 8yo has been forgetting his backpack at my place literally every other Wednesday when his mom picks him up. I've tried:

• Putting it by the door
• Reminding him 5 mins before pickup
• Letting him experience the consequence (forgot homework, got a note from teacher)

Mom's getting frustrated with me like I'm not prepping him properly. But he's old enough to remember his own stuff right? Or am I being too harsh here?