Not helping without proof of purchase

I bought a school shirt for my grade 1 son in January 2016. Last week I noticed that the shirt's colour faded. On Saturday 5 March 2016 I took the shirt to the shop. I was asked for the receipt, I did not keep it because I was happy with the shirt. I was told that the shirt will be shown to the manager on Monday. I she will let him phone me. Monday 7 March 2016 I realized at 15h30 that I haven't heard anything. I phoned Nicky (the manger), he also ask for proof of purchase which I don't have. He then said that if there were more complaints then he would know that there is a possibility of a batch that had a problem. I understand that. I then asked on my son's class whatsapp group and 3 other mother's complaint of their shirts fading. I phoned the manager today to inform him about this. But still he told me that he can't do anything for me without the proof of purchase!