I've never worn it and l picked out the cutest coat for my son made of Polartec fleece but l do not know just how warm it is. WIll it keep him warm in the winter?

pretty warm - even better when you layer it with a turtleneck.

Polartec is extremely warm and comfortable, but in my experience it is better for late fall than for winter. In sub-freezing temperatures, your son might still feel the cold through the coat.

toasty but you need to have something over the top that will keep the wind out.

The heat works but trapping air which is not a good conductor of heat so it traps the heat inside. If you do not put something on the top then the wind will take that nice warm air away.

It is brilliantly comfortable and warm (i have 4 jackets and shirts made of it), but he should wear something wind-tight and water proof. l would suggest a thin gore-tex jacket (or a warmer one if it gets cold where you live). l have a gore-tex coat, and in the winter, when we get -20 Celsius, l do not even feel the cold, even though l stay outside a lot. gore-tex and polartec fleece make the perfect combination.