Asphalt has less heat retention capability than sidewalk bricks. The asphalt is venting more of its heat into the surroundings than the bricks are, which is why it's cooler.
There's also very little traffic on his side of the street -- you'll notice there are almost no cars on that side, so it's more than likely he had ample time to get a temperature read on the asphalt before a car came along
Considering a lot of the NYPD straight up said they'd resign if he became mayor, I think he's got a pretty good chance of bringing change if he really wants to