sorry avocado, haven't had time to read the whole thread, jsut wanted to say briefly that learning how to practise I think is one of those things that have to grow a bit. Give him a bit of time to understand how to play a piece beautifully and smoothly. Atm he doesn't he really yet know what to listen for in music but he will pick it up from his lessons and hearing his teacher demonstrate. It will click into place.
If you are there at a lesson or at pick-up, could you ask the teacher how s/he suggests he practise? Thing is crazy mad hacking away at the violin and enthusiastically storming to the end of the piece x 38 doesn't really achieve that much. So when he is ready, he'll break it down and listen for background bow noise and all the rest but maybe not yet. If you play an instrument yourself, you could try to rmemeber how you practised as a beginner.
Otherwise if you don't know how to hold the violin correctly (left hand, bow hand, bow arm, posture, all the rest of it, I wouldn't step in too avidly). Problems hard to fix later.
What I would do: He is only 6, he 's just beginning with it, is be gently enthusiastic. Do not seek to control his practice by being too domineering would be my suggestion.
How about a game like hot/cold? I used to do this with dd and she loved it. Do you know how his hand is supposed to hold the bow and how the bow arm moves? if so, he can choose an open string. I think it will be A and hide a little toy somewhere in the room. You have to find it. When you get closer to it, he has to play forte, just increasing the volume; when you move away from it, more piano, so quieter etc. Important is forte is not pressed out of the string but drawn out with the bow. Listen for that. Piano is not shivery and shaky and needs a bit more push for an upbow. However don't lecture on it. should be fun. When he does it on the D or G string, his bow arm (elbow) has to go higher up. One string just hot/cold. I think he'll enjoy it and it will help a bit.