Kids clothes at baby mamas...

Okay so.. ive been w my SO for two years.. he has a 4 yo from his ex wife, every single time i buy him clothes /shoes etc, they go to his moms amd i never see them again and it gets on my fucking nerves because we have him half the week. This last time i bought him clothes, i bought him 2 sweat suits, 1 tshirt short outfit, 3 shirts and a batmam coat and a pair of nikes and i told him id prefer if he just kept the clothes at our house and tell his ex she doesnt have to send him w clothes anymore but he says its pointless to put him in the clothes if he has to take them off of him before he drops him off.. but he'll be w us for days at a time.. am i just being ignorant for getting annoyed w this whole situation? Or suggesting we keep stuff at our house? I know i dont have any living children so maybe im just misunderstood...