I've never experienced this with my child but I get neck spasms from time to time. They are just about the worst pain I can think of. You don't realize how much you move your neck when you move your body. It's part of the way we balance so every movement, even just lifting your leg off the bed hurts severely. There doesn't seem to be a reason for why I get them. They tend to just "happen". However, high levels of stress can trigger them too. I'm an athlete and to be down for the count is just unbearable for me. Regular doctors would give me painkillers and muscle relaxers and send me home. This only fixed the immediate problem (while I sat around in a drugged out haze) but they always came back. Now, any time I feel my neck muscles getting tight I go to my chiropractor. He's wonderful. I haven't had a neck spasm in about 2 years since I've stayed on top of this. I used to believe that chiropractors were just quacks until I met my believer husband. He made me go. Scared the doo out of me the first time I went.
Now we take our 6 month old daughter. Not that she's got too many issues but he did seem to help her Torticollis which is where one side of her neck was weaker so she compensated with her other muscles never strengthening the weaker side. This was caused from her being squished up in the womb. With exercise, baby versions, and the chiropractor she's not a "Torti" baby any longer. Her physical therapist said she was shocked at how fast she got stronger and better. It's scary to see your baby being manipulated by any doctor but in this case there isn't the snap snap you get with adults. My daughter even seems to like it. It tickles. Our doctor is one of the only ones that works on children. He works on his own grandchildren even though his son is a chiropractor too in the same office if that gives you perspective on how specialized you have to be to work on children. He's also works with the University Of Texas athletes and is their main adviser for chiropractic. His experience is worth it.
If this sounds like a choice for you he does FREE CHILDREN'S VISITS, I believe, on the second Tuesday of every month. (I put that in caps because it sounds like it's what you are looking for and if you were just scanning this I wanted you to see it. I'm not a walking billboard for Dr. Mike, just really happy with his services. ) You can call and get the exact details. Also, this time of year he runs specials like office visits for canned food. His info is: Dr. Mike Vandawalle of Vandewalle Chiropractic ###-###-####. Ask for Dr. Mike (his son (Dr. Alex) is a chiropractor in the same office and is also a Vandewalle so they are informal about using first names to differentiate)
What ever you decide to do you do need to get your child in to see the doctor, at least Dr. Mike, to see what is going on. Spasms aren't normal for a 6 year old. Dr. Mike will look at the bones and muscles and let you know his opinion before he ever does any kind of adjustments. He's just as good with the parents as he is with the kids. His visit is free so it might be what you are looking for and he's not going to load your son up on painkillers and muscle relaxers like the other doctors will for spasms.