What Causes Persistent Fever, Lethargy, Poor Appetite And Constipation In An Infant?

Hello.. My son is 21 months old,he started getting a fever 5 days back which spiked the second day, 103.5, but has been between 99-100 since last three days, but he is extremely lethargic, not playful at all n not eating anything n not passing stool n passes urine 4-5 times a day .. He has a cold n cough also.I just give him some juice, or glucon d water n some fruits forcefully...dr got his urine test done for protein n culture, routine report is fine, no protein..our household help was detected with tb 4 months back.. So v thought of getting chest x Ray done, which was clear...please suggest what should be done and what can be d reason for his lethargy, loss of appetite, n not passing stools n less urine... Even when d temperature is not much. Thanks