My 10 year old son is complaining of severe pain in the left lower section of his abdomen. it seems slightly swolen. he had a normal bowel movement about an hour ago. should i 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.