I recently went through this with my ex. i live in nj, where the weather can change drastically from day to day with the change of seasons. it got to the point where i was packing bathing suits, shorts, pants, short sleeves, long sleeves, sweat shirts, jackets, flip flops, sneakers, etc..on top of sippy cups, medicine, tooth brushes, diapers, toys, strollers, and even car seats for 2 kids. i was going out of my mind!!! the worst part was i wouldn't get back the stuff i needed for them, and he was not willing to go out of his way to return the stuff. it was FRUSTRATING and we were constantly at each others throats!
I kept asking him to get stuff for them for his house, but he wouldn't. i didn't want my kids to suffer by not sending what they needed, so i felt trapped to keep sending it. eventually i sent him an e-mail telling him i no longer wanted to keep fighting with him over stupid stuff. I made a list of things i would not be sending anymore, and it was up to him to get it. I started simple: sippy cups, diapers, wipes, toothbrushes...and gave him two weeks to stock up. the next month was strollers, etc. eventually clothes, and finally toys.
a year later i am only packing a set of clothes for them to come home in and their blanky's. i am less stressed, and we don't argue!
my advice: keep it civil, but be stern. figure out what he needs to get for your son, give him fare warning you will no longer be providing it. then give him time to get it. end of story.