School trousers too cold

[Aaviiii]

I'm hoping someone can help me DS1 is in Year 7.
The past week or so he's been going to school wearing pyjama bottoms underneath his school trousers because he is cold. As they are not ideal can someone suggest something else that I could buy.

[sashh]

Long Johns.

Or opaque tights, but not on PE days for the laughing at factor.