The Doppler Shift formula $f'=f(\frac{v \pm v_{obs}}{v \mp v_{source}})$ only works if the wind (or the medium that the sound is moving in) is constant. Therefore, if the wind is moving at a constant speed, change the reference frame so that the wind is stationary.
In your case, change the reference frame so that it is moving from the train to the boy at $30m/s$. This way, the wind is stationary in the new reference frame. Then, calculate the speed of both the observer and source in the new reference frame: $f'=f(\frac{v \pm v_{obs}}{v \mp v_{source}})=f(\frac{343m/s + 30m/s}{343+ 30m/s})=f=400Hz$
Note: If the windspeed changes, the frequency will be different as the reference frame will be changing all the time.