Trump's campaign has ushered in a whole new paradigm shift. He defeated 16 other Republican candidate with little or no advertising. What makes you think he'll lose to Hillary if he doesn't spend on ads.
Trump's advertising lies with his rallies. He is interacting with people on the ground. There is no need for him to spend millions on ads, on network televisions, although I think he'll cut ads after Labor Day.
He also does local TV in the rally towns