What Causes Nausea Along With Poor Appetite And Headache?

Hi , My 17 yr old son who has ADHD, Tourette Syndrome and has high functioning autism is experiencing nausea after eating, poor apetite, headaches for over a month. Last night he had a pain in his rectal area also that was so severe it made him drop to the floor. (He tends to have heightened reaction to pain) Normally he has a very healthy apetite and Lasagne is his favourite but he only had a very small piece and complained he was nauseated after eating it. He is not eating well and complains that his apetite is fussy. (Normally he is like a typical teen and cannot seem to eat enough to fill him).... He looks pale and is losing some weight but he has also had some weight loss that was resulting from having his wisdom teeth out the last week of January.