Jeremy Heidt of Tennessee Emergency Management said a family of four was stuck in high water in Hickman County. When they got out of their vehicle, the rushing water swept the boy away, he said.
Three people died in Tennessee and Arkansas on Thursday in weather-related incidents.
In Lawrence County, a 72-year-old man died when he tried to drive across a flooded road and his car was swept downstream and submerged, the Tennessee Highway Patrol said.
In Jonesboro, Arkansas, a 74-year-old retired police officer was killed when a tree fell on his house, reported CNN affiliate WMC.
In Black Rock, Arkansas, a tree fell on a van and killed a woman inside, said Brandon Morris of the state emergency management agency.
Memphis took a hit Thursday from storms that had rolled eastward from the Great Plains.
The Memphis Light, Gas and Water's website said about 42,000 customer lost power at one point due to bad weather.
Delta Medical Center was without power and had a gas explosion that caused a two-alarm fire. CNN affiliate WCM, quoting a hospital spokesman, said nobody was injured.
The weather caused play to be suspended for several hours at the FedEx St. Jude Classic golf tournament on Thursday, according to the PGA website.
Heidt said other Tennessee counties reported weather problems.
In Carroll County, some hallways were flooded at Baptist Hospital.
Madison County had power outages at the Regional Health Department and its emergency operations center. Lightning knocked out power at the Henderson County 911 Center, forcing authorities to use a backup system.
More bad weather is expected Friday in Memphis, with the National Weather Service predicting a 70 percent chance of precipitation and up to 2 inches of rain. There's a 60% chance in Birmingham, Alabama.
CNN meteorologist Sherri Pugh said severe weather could last until Saturday in the South and Great Plains, with heavy rain, hail, damaging wind and flash flooding possible. Isolated tornadoes are possible, too.
In parts of Nebraska, people were cleaning up from the hailstorms that struck on Tuesday.
As expected, bargain hunters flocked to Woodhouse Family Auto in Blair to buy some of the 4,300 cars damaged by hail, reported CNN affiliate KPTM. The hail smashed car windshields and dented the bodies. Dealership officials estimated the damage at $162 million.
Car prices were determined on a case-by-case basis, with one buyer saying he picked up a Chrysler 300 with broken windows, a broken door handle and lots of dings for $31,000, reported KPTM. The car normally sells for about $54,000, the buyer said.
Copyright 2015 Nexstar Broadcasting, Inc. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.