Frustration with G Scales 4 octaves

My son's been working on G scale 4 octaves for several months. He is struggling with the last shifting. My observation is

1. His hands are small (as of now)

2. His thumb position is not stable

3. His pinkie is short

All of the above combined he is having hard time placing his fingers in time for the high register notes. I watched his teacher demonstrate the scale. At the last shifting, his thumb is placed right where the neck and the body of the violin meet. But if my son does it he can't reach his pinkie where it supposed to be. His teacher wants him to find most comfortable and natural placing of this thumb for himself. Everyone's fingers are made differently and so as violin. So he didn't want to impose one set position that might not be right for my son. Now, he hasn't found yet. He gets really frustrated. He places his thumb about an inch below from the neck of the violin on the side (rib), but his thumb moves while he tries to hit the notes. I think that is causing his unstable intonation. This morning, I pointed it out. I told him if he feels comfortable placing his thumb where it is, it is OK, but he needs to make sure his thumb is secure so the sound will be secure. Ahhh, he got it. It was a light bulb moment which happens to him ever so often after numerous frustrating attempts. I left him alone for him to figure this out on his own. I'm sure he'll find the answer soon.....