Hi I.,
It's not that uncommon for kids (or anyone) to develop a wry neck (torticollis), which is basically a muscle spasm in the neck. It can be very painful and frightening but is not usually an indication of anything worse. One thing you should definitely NOT do is try to manipulate the neck roughly.
As with any muscle spasm, a good course of action is to treat with heat and gentle massage. Since it is usually the great muscles of the neck that are involved, massage gently downward from the skull down the neck, into the shoulders and back. Another good trick is to apply a warm compress and have your son lie down on the opposite side from the bend with a rolled up towel under his neck for support and no pillow. This give support to the neck muscles and allows the spasming muscles to relax gently. Make sure your son supports his head with his hands when he gets up so that he doesn't aggravate the spasm. It is also best if he rolls over onto his knees before rising, as this keeps the head in a more neutral position. Once he can get his head into a more normal position, wearing a cervical collar for a couple of days can sometimes help to keep the muscles from respasming. Usually, this isn't necessary.
The only thing I'm a little concerned by is how long this has been going on for, but 3 days is not overlong yet. Sometimes wry neck can be an indication of an underlying infection in structures around the jaw, such as the ear, a tooth, the tonsils, or the adenoids. Get this checked out if he has any signs of fever, malaise, or swollen glands around his neck and chin or under his arms. Wry neck can also be caused by nerve root compression, in which case you will definitely need to see a doctor and physical therapist.
I hope he feels better soon!
R.