Your main problem is that you don't have enough state variables. You have these two equations:
x'' = 0
y'' = -g
That's two 2nd order equations, so you need to have 2 * 2 = 4 state variables. The state variables you need to integrate are x, y, Vx, Vy. But you only integrate two of these state variables, x and y. You have no code for integrating Vx and Vy. You need to add code for integrating Vx and Vy.
You also are using the solution of x and y integration as the derivative functions, which is incorrect. For a Runge-Kutta method, just use the derivatives. The code would be patterned after:
x' = Vx
y' = Vy
Vx' = x'' = 0
Vy' = y'' = -g
You also use g for both gravity and a function handle, so to avoid confusion you might want to change one of these.