My 15 Year Old Son Had An Appendectomy Last February.

My 15 year old son had an appendectomy last February. Around midnight last night he began having pain in the same general area, said pain is similar. Pain is slightly higher on the right side than his incision was and the area is tender to the touch. He thinks his urine looks darker than normal but there is no blood in the urine. The pain has gradually increased but he was able to go to school--didn't tell me of the pain--and is complaining of it now. Oral temp is 99.5. Any ideas of what this could be?