hi..
your son has fever,
joint pains, vomitting and weakness..
this looks like viral fever/arboviral infection.
see your physician and get the following investigation..
CBC,
Malaria test,
widal test, Dengue
antibodies test, and urine examination..
he may need hospitalization, since he has continuous vomitting and not able to eat or drink.. needs IV fluids..
all the very best..