The given symptoms are paleness , back pain
& loss of appetite ..
As he is 3 years old :
Kindly check : Blood test
HB/ CBC / ESRSGOT
Kindly start with daily intake of milk 100 ml (unskimmed) twice a day + min 3
main course meals daily
If there is no infection he will be active in no time & also check where he sleeps it should be flat surface not the uneven surface ..
Will require time to get the results min 15 days
Revert back the response will be appreciated