Police say Terry Smith was discovered missing from his residence in the 33000 block of Helen Lane sometime between Saturday at 7:30 p.m. and Sunday at 10:30 a.m.
Police and volunteers, along with a helicopter and bloodhound, scoured the area all day on Sunday, but could not find the boy.
His parents say he takes medication twice a day. The last time they saw him was right before he went to sleep.
"She checked on the kids, thought he was in bed. This morning when I woke up at 8:30 to groom a dog, he was not in bed," said Dennise Bugna, partner of the child's mother. "We just want the family back."
Smith is 4 feet 8 inches tall, 76 pounds and was last seen wearing blue basketball shorts.
Anyone with information is urged to call the Menifee Investigations Unit at (951) 210-1000.