Another user in the dark web - a hidden part of the internet where guns, drugs and child abuse images are traded - alerted police, a spokesperson said.
The boy's body was subsequently found in the cellar of a house in Herne, North Rhine-Westphalia.
Police say the 19-year-old suspect may be armed.
Searches are being carried out in towns near Herne, which lies in the Ruhr area, a heavily industrialised region.
Reports say the suspect, who is on the run, is a martial arts enthusiast.

The small aircraft came down in a field at Steve Powell's farm at Cwmbach, Builth Wells, Powys, just before 12:00 BST on Sunday.
The men from Holmes Chapel, Cheshire, have been named locally as Scott Penlington and Nick Jefferies.
Mr Powell said the aircraft appeared to have "fallen out of the sky into our field".
"It is a terrible tragedy and we've never had anything like this before," he added.
"The police are examining the wreckage but we don't know exactly what happened."
Dyfed Powys Police said recovery work has now been completed and the emergency services left the scene on Tuesday.
A spokesman for the Independent Air Accidents Investigation Branch, which is leading the investigation, said the wreckage was being recovered to its headquarters at Farnborough for a more detailed examination.
He added safety recommendations may be issued to prevent similar accidents in the future, if any issues were identified.
One Direction singer Harry Styles is a family friend of Mr Jefferies after he once dated his daughter Emilie, also 22.
They remained friends with her father taking him up in his single-engine two-seater Cosmik plane.

Sam Amin, 18, fell into the river after a night out in Bath last September.
Since then safety railings have been placed along the stretch of river where six people have drowned since 2009.
Laurence Tindall, who will design the memorial, said: "We want people to think quietly about the people who lost their lives in the river and how young they all were."
The monument will be placed at the spot where Mr Amin died and will be part of the revamp of Bath Quays, near Churchill Bridge.
Mr Tindall is in talks with Bath and North East Somerset Council to settle on a final design.
A spokesman from Bath and North East Somerset Council said: "The council is working with Sammuel Amin's family to define the most appropriate form and location for the memorial, which is expected to be installed within the new Bath Quays Park by the end of 2016."
Attempts will be made to contact other bereaved families to see if they want inscriptions dedicated to their loved ones added to the artwork.
"We can't approach the families directly, it's a very sensitive area - we have to approach them through the police formally," said Mr Tindall.
"I'm sure it will give them a great deal of comfort to know that there is something permanent, so it will not only remember him but the other young men who have died and to help people to think about river safety and take due care when they're beside the water."
The project is expected to take at least a year to complete.
The family has now opened an appeal to help pay for the memorial costs and the council has agreed to pay for the installation.
Mr Amin's memorial service takes place at St Michael's Church, Twerton on Saturday morning.
A manhunt is under way in Germany after a man allegedly stabbed a nine-year-old neighbour to death and uploaded a video boasting of his deed to the dark web.
[NEXT_CONCEPT]
The owner of a farm where a microlight crashed killing two people has said the men had "no chance" of survival.
[NEXT_CONCEPT]
A monument will be built in memory of a student who died after falling into the River Avon.