An Amber Alert has been issued for a missing 8-year-old boy.  Peel Regional Police say Matthew Zhang was last seen by his father yesterday morning.  His father dropped him off to see his mother at Mississauga Valley Boulevard.  Just before 5pm, the mother, Wei Chen, refused to return the boy.  Police believe the victim’s mother may have took off with him.  Matthew is described as 5-feet tall with a thin build and black hair.  The mother drives a grey Honda Accord, licence plate A-V-A-M 0-0-9.  She is 5-foot-6 with black medium-length hair.  Police are concerned for the boy’s safety and say the mother doesn’t have custody and can’t care for the boy.