The powdery mildew on your Oak can be controlled by spraying with a fungicide. There are lots available, just check the manufacturers label to make sure you choose one that is suitable for clearing powdery mildew on ornamentals. Except that I would not prune your tree as this will promote lateral shoots to form and would ruin the shape of your young oak. It will probably defoliate when it is ready to so just make sure you collect up the infected leaves as they drop. I would also repot it in a deep container using soil based compost such as John Innes No.3, and make sure that it is kept moist at the roots. Container grown plants are particularly susceptible to powdery mildew because they are reliant on us for their water source. Try not to position it in full sun all day long. A semi shaded site will slow water loss and reduce plant stress. Look after it, and it should be fine next year.