Try a fungicide spray on it.

And why did you not ask your parents? Himself suggests you put it somewhere that it will get a lot of air around it - not easy in your garden and I assume the youth would wreak if you put out front. Are all the leaves infected? If not simply remove the ones that are and bin.

More info via Ilex:

Good air flow and increased sunlight will kill the fungi, you should cut out any severely affected leaves/shoots and be careful to disinfect your cutting tool (boiling water works well) otherwise you risk spreading the fungi to other plants.
Mildew is killed by sulphur & you can make a home-made fungicide from crushed garlic - put it in a food processor with some water, add a few drops of vegetable-based soap (for adhesion) and then spray it on all effected leaves.
Bicarbonate of Soda also kills Powdery mildew - 1 teaspoon dissolved in a litre of water, again with the soap for adhesion.

Thanks, tree is out at the back of the garden by the back fence, so plenty of air around it, will have to get some veg based soap to spray the leaves.

Good luck lad! If it all goes pear shaped grow another tree!

would rather it went apple shaped, lol

Well yes... and come to think of it you are hardly a 'lad' anymore!