The smash happened on the M65 in Lancashire around 8pm on Thursday.
The driver of a Mercedes C220 and his front seat passenger, a boy aged eight, were both hurt.
The youngster was taken to Manchester Children’s Hospital with head and leg injuries. The driver was treated for minor injuries.
Police say the flat-bed truck involved is believed to have been stolen from the Sheffield area.
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The driver fled the scene on foot and is now being sought.
Sgt Finn Quainton said: “My thoughts are with the boy who I wish a full and speedy recovery in hospital.
“We are actively trying to track down the man that was seen running from the truck.
“Not only do we need to speak to him, but he may also have been injured and could require medical assistance.
“If you have any information about him, or indeed the incident itself, please make contact with us.”
The crash happened near to junction 4 of the motorway near Darwen.
The two vehicles collided, sending the car into the central reservation and then back into the path of the truck. Both casualties are from Aughton near Ormskirk.
Anyone with information should contact police by calling 101 and quoting incident reference LC-20170517-1422.
Alternatively they can contact the independent charity Crimestoppers on 0800 555111 or online at crimestoppers-uk.org