Why Is My Son Sick All The Time?

Why is my 14 year old son sick all the time? In the last 4 months he s had scarlet fever twice, stomach flu , sinus infection, allergies, and now something else that includes nausea , vomiting, and diarrhea . He s been sick with everything nonstop since the stomach flu and only had a good 2 weeks of normalcy in between that and scarlet fever. He s normally very active and eats a ton. He hasn t had much of an appetite at all. he s lost 15 pounds since the beginning of the year too. I m worried.My son is 14 years old he weighs 115 pounds he s about 5 9. He had his adnoids removed when he was 5. He suffers from seasonal allergies. He is currently on amoxicillon and flonase and has been for 8 days now.