Hi thanks for your question.
Your six year old boy most probably has developed gastroentritis that is the infection of the stomach and gut.Abdomeninal pain and loose motion
could be due to gastro entritis.Headache could be due to fever
.Give him ORS
(oral rehtdration therapy).Plenty of water and juice.For fever and headache you can give him ibuprofen
or pracetamol.Gastroentritis in childern is usually viral and get settle of itself .Consultation with childeren physician is advisible.
Hope this answers your question.