My 3.5 yr old is learning the violin. he plays well, except for the posture. He would lower his neck and tilt his head up. His violin is not straight across but tilting downwards at an angle. I wonder is it a problem with his shoulder rest, or is it just a bad habit?

It could be either. This problem is common in violin students, and becomes harder to correct the longer it is allowed to continue. Make sure his back is straight whenever he is playing (often a drooping violin is caused by a slouching back). Have him stand straight and hold up just his violin (not the bow). Let him relax his hands down at his sides (since the violin should be supported almost entirely by the head while playing.) If the violin is still tiled downward when he does this it might indicate that he physically cannot hold it straight. Then it would be a problem with the shoulder rest. I have been playing violin for about 12 years, and have always had what seems like the same problem. I was just told to stand straighter, but finally my teacher realized that i just have a long neck, and that my shoulder rest couldn't go high enough. This may be your son's problem too. You could take his violin and existing shoulder rest into a music store, and ask them if you could try out other brands of shoulder rests. (The kind i have now is called a Wolf. It is able to be raised quite high and has fixed my problem, but was slightly expensive.) Have him do the same thing i suggested earlier to see if they correct the problem. If the problem does not improve, then it would be something wrong with the way he's holding the violin. The most sure way would be to ask a violin teacher. (He should probably be taking lessons anyway, because any bad habits developed at a young age become extremely hard to get rid of later.)

Thks for the advice! Will sure do as you suggest..