Brief Answer:
blood test will help

Detailed Answer:
Hello

Thanks for the query

I can understand your concern.The most common cause of fever in this age is some viral infection.But in viral infection fever usually shows a declining trend after 3-4 days.
Fever persisting for 7 days in the absence of flu symptoms warrants the blood test.
I would like to get complete blood count,c reactive protein (raised in bacterial infection or other inflammatory disorders),malaria kit test and also smear for malarial parasite,widal/typhidot test (to rule out typhoid) and also blood culture to find out the causative organism causing this.
I would also want to get urinary examination done to rule out urinary infection as a cause of fever.

Till the reports come give him antipyratic.You may start broad spectrum antibiotic after discission with your doctor,once the sample taken.
In most of the cases the cause can get determined with all these.If all these test don't provide a clue and fever persists even after giving antibiotic further investigations may be needed like chest Xray, ultrasound abdomen or other blood tests.

I hope this helps.
Please let me know if you have any more query.
Regards

---