There's a few differences. First, Brexit was a simple majority vote. The US Presidential election is won by amassing 270 electoral votes. Second, Trump just doesn't have the coalition of voters needed to make it close, like Brexit was always was. He can lose entire states (like Ohio and Virginia), and valuable electoral votes, simply because his numbers with minorities are shit. And women. And like I said above, he's behind Romney's 2012 numbers even among those who support him.
His path to victory is narrow. A lot has to change for him to tighten this up before November. Hillary needs to have horrible debates for him to even get close going into election day.