Wondering If You Could Please Help. My Son Had Pneumonia

Wondering if you could please help. My son had pneumonia and ended up in Hospital for 13 days. The infection has now cleared but his body temperature is low (35.5-36). Having gone back to the hospital yesterday, blood test came out OK and so did the X-ray. Thyroid was also checked and was OK. I cannot understand why he feels so cold. Been 48 hours since his temperature dropped