Co parenting school supplies help

Dear IMs

I share care of my son with his father 50/50 at th start of the school year I purchased everything for my son for school except his sports shirt.
I bought 3x pairs of uniform so that he had one to wear one spare in his bag and one spare at my house...
Now recently it's ended up with all 3 pairs at my sons fathers house (my son wears the 3rd set to school) I have reminded him over and over to pack the spare set in his bag and the other set to be at mine...

How does everyone else do it?
Do you just each buy 2 sets or?
I'm getting frustrated with having to repeat myself all the time!!!