Violin can sound nice quite quickly. The teacher will be telling your son how to do it and it really does help if you can praise specifically what he is doing well and suggest just one single area to work on.
eg "Well done! You have got the rhythm spot on and a lovely straight wrist. Give your wrist a little rest then let's hear that again and see if you can keep your bow straight all the way through like your teacher has written in your practise book."
Here's a checklist for a nice sound. It helps a lot to practise in front of the mirror and take each of these sections one at a time:
Feet apart, left foot forward, knees able to bend.
Supporting violin with the weight of the head, the left hand helps a little.
Violin parallel with the ground.
Holding bow as teacher advises, with soft thumb and curved little finger.
Bow resting on the strings, not crushed into them.
Draw the bow straight, at a right angle to the strings. (Try it different ways to hear how much better the tone sounds with a straight bow.)
(He should be able to get a nice sound out of the open strings like this.)
Left wrist straight, violin resting on V between thumb and first finger, not in palm of the hand which would give a hairy out-of-tune sound.
Left fingers standing straight up on the strings.
(This requires strengthening some muscles in the wrist, best to practise for one or two minutes with a perfect wrist, stop, rest, repeat. Encourage your son to quit while he is ahead with that and his wrist will soon be lovely and strong and he will be playing nicely.)
Hope this helps. Let us know how it goes.