Joshua Dale, 14, from Nottingham was knocked off his bike in 2012.
Friends and family raised £25,000 for a skate park that Joshua had dreamt of building, and it opened a year later.
The county council, which also part-funded it and managed the project, said once built, the family's charity was responsible for maintaining it.
Joshua's mother, Faith Dale, who runs the Joshua Dale Skate Park Fund, said they had been "forced to close" it because it is "no longer safe to use".
She said the ramps were rotting with 28 large holes appearing on them, there was a build up of moisture on the structures and "inferior plywood" had been used.
"It is incredibly sad," she said.
"I would love to come here in the summer and see it full with children of all ages... but we don't feel it is a safe structure anymore."
She accused the county council of carrying out "an inferior build" and said they had been told repairs would cost about £28,000.
Nottinghamshire County Council, which had put £50,000 towards the area, said it was "concerned about the allegations" it had not been built to a high standard.
In a statement the authority said: "We built a quality design that has been used across the county for the past 10 years and maintenance for the park, once built, lies with the user group."
The council said it would have a discussion with the charity and contractors.
A skate park built in memory of a teenager who died after his bike was hit by a car has been shut after three years because it is "unsafe".