Regarding the “let’s laugh at the fat kids again” tag. It’s not about that.
I just find it a little depressing that a retailer the size of Tesco sells Children’s clothes that will only fit a larger child. Clearly it makes more economic sense to them to do it that way. Either that or this particular range of trousers are poorly sized, but it isn’t the first pair that haven’t fit him. My nephew (4 years old) is the same. He’s constantly wandering round with his bum showing because his pants are too big on the waist.