If he's coasting on a flat road, then there is no acceleration ...
his speed is constant and he's moving in a straight line.
No acceleration means the net force acting on him is zero.
Horizontal forces add to zero:
Nothing pushing him from behind.
Nothing pulling him from in front.
No horizontal forces.
Vertical forces add to zero:
Force of gravity pulling him down, equal to the weight of him plus the bike.
Equal normal force, pushing him up, exerted by the road.