Size of clothing is different from what it is on the label

I had a complaint regarding a product and has had no response from their customer care. I bought my son a babygrow from Naartjie a few months ago and had it in his closet as I kept the outfit for his birthday, the age is for 12 - 18 months. I bought the outfit for him to wear on his 1st birthday, which was last friday. He weighs 9.60kg and the babygrow just fit him (I am unhappy about this as he won't be able to wear it after 1 month or less. I would like a replacement as I feel I wasted my money if my son is unable to wear this outfit again. It did not shrink as I did not wash the outfit before he wore it.