Adam Owens, 17, was discovered on Monday, lying on grass outside a house in Bristol Park in the Westwinds estate, Newtownards.
He was taken to hospital by ambulance but pronounced dead a short time later.
His parents said the drugs transformed him "from a happy intelligent young man into a suicidal teenager".
His father, Carl, and his stepmother, Dawn Carlile, said the drugs changed their son so that "he didn't care about anything" and lied when they tried to help him.
It is understood Adam had been using the drugs for three years and his family had been trying to get him help.
Police are investigating the death and a post-mortem examination is to be carried out. But his parents believe legal highs killed him.
In July 2013, Sean-Paul Carnahan, 22, died five months after he had been admitted to a Belfast hospital with a brain injury.
Mr Carnahan, from Beechmount, west Belfast, had tried to take his own life.
He had been a regular user of legal highs and was left brain-damaged by the suicide attempt.
His mother, Tracy, said that after he took the drugs his personality began to change.
She said he had made a number of suicide attempts, with one of them resulting in him being "severely brain damaged".
Legal highs, officially known as psychoactive substances, are synthetic drugs that can be bought online and, sometimes, in shops.
They are often created in laboratories in the Far East and are rushed to European and other markets before authorities have time to ban them.
Several deaths in the UK have been linked to legal highs.
More than 200 of the substances have been banned since the coalition government came to power in 2010.
The parents of a teenager from County Down who was found dead have called for a ban on "legal high" drugs.