Hi,
Don't worry.. As these tests may have false negative reports. In other viral infection, malaria etc may cause low platelet count.
You should get it investigated in detail.
Complete blood count, Dengue NS1 antigen and IgM antibody and malaria antigen test, Blood culture.
Temperature charting and Tab Paracetamol 500mg thrice daily. He may also require antibiotics.
Consult if you have any queries.
Next Steps
you may also refer my article on dengue for more information.
"Dengue Fever : Don't Panic, Know How to Deal With It" by Pratyush kumar mishra - https://www.practo.com/healthfeed/dengue-fever-don-t-panic-know-how-to-deal-with-it-29261/post