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.

Henri van Breda, 21, is also charged with the attempted murder of his sister who survived with severe head injuries.
Mr Van Breda said in a statement that the murders were carried out by an axe-wielding intruder.
No arrests were made in this horrific case for 18 months until the accused handed himself over in June 2016.
Mr Van Breda's sister Marli is expected to be a key witness during the trial in Cape Town.
But there are concerns about how much she remembers about the night of the murders as she suffers from amnesia. She was 16 at the time of the attack.
The run-up to the trial has attracted international interest in the story of a son born into privilege allegedly unleashing a brutal attack on his family, whose fortune was estimated at $16m (Â£12.5m).
In a statement read out by his lawyer, Mr Van Breda said he was in a toilet when he saw a man wielding an axe hacking his brother Rudi in bed.
His father, who then burst in, "trying to tackle the attacker" was also struck by the alleged intruder, he said.
"I also recall the attacker was laughing", he added. His mother and sister were then assaulted.
Mr Van Breda is also facing charges of tampering with a crime scene.
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".
[NEXT_CONCEPT]
A man accused of killing his parents and older brother in a frenzied axe attack in Stellenbosh in South Africa has pleaded not guilty to murder.