My 6 years old has symmetrical rash on his low part of chest /abdomen it is small sized red rash that not itches or doesn't induce pain what can it be?

Many possibilities. Many possibilities include reaction to virus (ie: viral exanthem), medication induced reaction, bite reaction, other rashes of childhood. Best to see dermatologist for diagnosis and treatment.
Call pediatrician. This could be lots of things including viral, bacterial, allergic, immune related, contact rash, other. Call doc, needs to be seen to be diagnosed.