Ryan Davies and his mother, Alison, 40, had been missing for six days from the Greater Manchester area. Ms Davies, who had left a note saying she intended to take them both to the Humber Bridge so that her family "would not have to worry anymore", has still not been found and police have urged her to get in touch.
Coastguard officers recovered the body of the boy from the river Humber at Swinefleet Haven, near Goole in East Yorkshire, shortly after 10am on Sunday, after being alerted by the skipper of a pleasure boat.
The body was taken by the Humber rescue vessel to the nearby Blacktoft jetty and then removed to a mortuary in Hull where a postmortem examination took place yesterday. Detectives are treating the death as suspicious.
Police in Greater Manchester fear for the safety of Ms Davies, who suffers from depression but does not have her medication with her.
She and her son, who also received medication for his autism and had a mental age of seven, were last seen at her mother's house in Marple near Stockport at 6pm on 11 April.
In her note, she explained that she felt that she had failed as a mother.
The discovery of Ryan's body follows an appeal made by Ms Davies' sister at the weekend for her not to harm herself and to return him.
"I think she thinks she's a burden to us and that to save us all the worry, she'll take herself and Ryan away and we'll never have to worry about her again," said her sister, Lindsay Cooke.
Ms Cooke wept as she urged her sister to come home. She said: "We're sorry if we have not supported you in the way you wanted - but we do love you and Ryan. We will do all we can to make it better in the future."
Detective Superintendent Colin Andrews of Humberside Police said officers were desperate to find Ms Davies. "We're very concerned about Alison's safety," he said. "She's been missing for a few days now. Given the fact that Ryan is dead, then there has to be real concern that Alison may also be dead." Ms Davies is described as white, 5ft 4in tall and of slim build. She has shoulder-length brown hair and speaks with a Southern accent.
The tragedy comes four and a half years after Helen Rogan, another mother who was struggling to cope with the needs of her autistic son, threw herself and the 11-year-old boy 180 feet to their deaths from a railway viaduct at Hownes Gill, a beauty spot near their home in Co Durham.
Ms Rogan, 38, a single parent, had feared that her son, Mark Owen Young, would be taken into care. Before jumping, she gave him an overdose of prescribed tablets at their home in Consett, Co Durham, took the same amount of pills herself and slashed both their wrists with a razor blade.
In 2002, an independent report into the tragedy, commissioned by Co Durham child protection service, called on the range of agencies involved in child protection to develop a "despair-proof" support system for children and young people "with complex and challenging behaviour".
It urged help for carers, many of whom were likely to be less qualified and well informed than Ms Rogan, who was an occupational therapist.
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