If all the rest of the children in his competitive region are using poly, then he'll be at a distinct disadvantage if he doesn't use poly. I can at least appreciate his desire to be able to "run with the pack" and not get left behind.
Be that as it may, at 12 years of age, differences in shot placement is probably not due to two different brands of poly string. I'm sure it has more to do with technique. I'm sure he's being coached, correct? You should be speaking to his coach about why he's getting such different results with different strings.
Have two racquets strung with these two types of strings. have the coach observe his footwork, timing and hand eye coordination. If they are precisely the same, then ask the coach for his professional opinion as to why he's getting such different results.