My 10 year old son has severe pain in the left bottom of his abdomen. it seems a bit swolen. he says if he moves around and adjusts his position, the pain is better. do i need to take him to the er?

Some basics: Severe is a subjective label but suggests an urgent surgical problem. The location cited is where the last section of large intestine stores poop before exit. Swelling there suggests a large mass of constipated poop. Rarely the appendix is found at that location. If the kid can walk around or jump up/dn without pain it is likely not surgical. If it causes pain have him seen now.