Dad Darren Roberts says the European American Designs Sunscreen Lotion he bought at the discount store had failed.
He says he regularly applied it to his son Liam, but the youngster was still left with blisters and sores when out playing during the recent warm weather.
Darren, of Clwyd Street, Shotton, Flintshire, said: "It wasn't a particularly hot day, just a normal summer's day.
"Liam was out playing in the paddling pool and we put cream on him every 10 to 15 minutes.
"When he got up the next morning, he was covered in blisters and sores.
"We had to have the biggest ones drained and dressed.
Some of the blisters little Liam was left with after using the sun cream
"He was in so much pain he wouldn't let us touch him and at one point we had to sit in bed with him to hold him up."
Darren said he took his young son to be treated at Holywell Community Hospital as soon as he noticed the damage to his skin.
"I was devastated for him, it was horrible, I've never felt like that in my life not being able to do anything for him," he said.
Dad Darren bought the cream from discount store Poundworld
Darren, who also has a 10-year-old daughter Lucy, said: "I was horrified, I bought it to protect my son and it didn't do anything at all. I was disgusted.
"We expected it to work and it failed us.
"I rang head office immediately and they said they had used it and there was nothing wrong with it."
Despite the harrowing episode, Liam is on the mend according to his dad.
Liam was left with agonising sores on the top half of his body
"He's not 100 per cent yet but he's getting there," Darren said.
"He's back running around in the garden.
"When it happened he couldn't move, bend his arms or lay down."
A spokesman for Poundworld said: "We've been in touch with Mr Roberts so that we can investigate. Product quality and safety is of primary importance to us at Poundworld and we have alerted our supplier, EAD, about this issue.
"We hope that Mr Roberts' child makes a speedy recovery."