Good day, my son of 4 woke up this morning and was screaming of pain in his foot. he can't stand on that foot and can't walk. what can that be?

Can't say: Pain implies an injury. Whether this is due to a bug bite/trauma/infection will require an exam & possibly x-rays/blood tests.

See your physician: Pain that is so severe that it inhibits weight-bearing suggests significant illness or injury. It merits being seen by a pediatrician or orthopedic doctor in person so that serious disease can be ruled in and treated or ruled out.