10 Yrs Old Has Vomiting, Nausea, Stomach Ache, Sore Throat, Headache And Fatigue. On Amoxclav. Suggestions ?

My son 10 yrs old has been sick for the last 18 days. He started off with what I thought was the flu because his sister had it. Vomiting, nausea , stomach ache, sore throat , headache and fatigue . She gotten over it after 4 or 5 days but he stayed sick. He is a hyper kid and for the last 3 weeks he s been just laying down everyday. He has a constant headache and says he feels a pulling squeezing painful in his stomach area above his belly button. Nothing I give him helps either pain to subside or be relieved I have taken him to the er 4 times and his doctor 2 times. An ultrasound was done of his stomach they said all his organs look good. They did a CT scan of his head and said they saw nothing. He is on amox clav for what the doc called a sinus infection . They did a blood test a week ago and said he had traces of mono but did not have the virus. Please help he is hurting constantly with headache and stomach ache