Poor clothing and service from Montclair Store and customer care

Just a warning. Purchased a pack of 3 boxer shorts for my son on Monday size small from Mr Price Montclair. Took them home and he put the 1 pair on over his underwear for Monday night, no problems. Tuesday he put the second pair on and within 20 minutes it had left red marks on him because it was tight around his waist. Went to take them back to exchange today, cashier referred me to manager who looked at them, said the bigger pair was stretched (which means someone else wore them before and stretched them then returned???) but was going to exchange until cashier added her 5c and said they had been worn so can't exchange. Does this mean they were going to take the pack of 3 boxers that all say small yet are different measurements back on the shelf for another customer to purchase because you are not allowed to open package in shop and check prices? I have got pictures that clearly show the different measurements on each pair and its not just the waist that's bigger the whole side is bigger. I buy from Mr Price monthly but will be closing my account now due to this.