My son is complaining of pain on his left side of stomach even when he moves what is the cause?

If severe, get help: There ae lots of things in your abdomen. On left side is colon, kidney, intestines, spleen, testicle. In some kids the appendix is over on the left side (right side more common), his guts could be twisted inside themselves. If he is in severe pain, made worse by movement, this may be an emergency, or something very simple like constipation. But severe abdominal pain always needs evaluation, now!