Re: The battle of the bulge
Does the school object to him not having a coat? Will he not be aloud outside with the other kids, etc? <-- if that is the case, could you treat wearing a coat when it's cold similar to not running out into a busy street: it's not an option or discussion point.
You could teach him to read a thermometer and put a big mark on there for when a coat is needed, so he can be more in control, but make it a fact, not a choice.
A (usually) happy wife mama to my BOGO girls, DD1 July 2010 and DD2 Dec 2011.