Shoppers watched in horror as Ryan Hooley fell from one floor to another and landed on some clothes rails.
The youngster, from Beeston, Nottingham, area, was rushed by paramedics to hospital after the accident in the Nottingham store at 2pm yesterday.
He was found to have fractured his skull in five places and doctors have placed him in an induced coma. He is in a critical but stable state at the city's Queens's Medical Centre.
Ryan's uncle said: ""We don't at this stage know exactly how the accident happened but Ryan's mum and dad are both in pieces.
"It's perhaps surprising Ryan hasn't suffered more broken bones, I have worked at height before and known people die falling less distance."
He said that police had been in touch with Ryan's mum and dad and doctors had told him Ryan could be taken out of the coma within 24 to 48 hours.
A witness to the fall said: "I saw him fall from one floor to another and he landed on some clothes rails. It was awful."
A Primark spokesman said: "The safety of our customers in our stores is of paramount concern to us. We are now working with the relevant authorities to investigate this incident."
Nottinghamshire Police said the Nottingham City Council is leading the investigation and The Health and Safety Executive are also involved.