Quoting Honda450:" We do 50/50 so he sends them some weeks two days to school and others three. Which yes he sends them ... [snip!] ... the pants look like capris. The shoes are chewed up by their dogs and I can see their feet because their socks have holes too!"
Has the school expressed concern to you about it yet? I mean my son's teacher called me the day my son came to school in a pair of to short of pant simply because it was laundry day and it's not like i was gonna send him to school in his Pj's. What your ex is doing is wrong, and my only advice is to buy your son's clothes and keep them at your place and send them back in what your ex sent them to you in.
I honestly would document the outfits he sends them back to you in because that is not ok to do.