Family friends in Brussels said that Mohamed was missing his mother and becoming increasingly nervous as the day he was supposed to start school approaches.
Labour and Liberal Democrat politicians say the case is further evidence of the Home Office's "hostile environment" culture, which has been blamed for scandals such as Windrush and the erroneous sending of deportation letters to EU citizens after Brexit.
The uncle of the boy, who was marched away from the plane home by police officers almost two weeks ago, is former Premier League footballer Kamil Zayate.
Keita met Mohamed's father Mangue Bangoura, while visiting her sister in Sheffield. The couple are no longer together but Keita decided to build her life in Britain where Mohamed has spent his entire life. Bangoura is of Guinean origin but is a British citizen who was settled in the UK at the time of his birth.
In a letter to the boy's mother, and seen by the Telegraph, the Home Office argued that Mohamed should never have been given a British passport in the first place and was given citizenship by mistake. The passport "should never have been issued", the letter said.
The Home Office letter was first sent in March but Keita says she never received it. It said Mohamed did not qualify for British citizenship because Keita was married to her estranged husband, who lives in Guinea.
But the Home Office said that Keita's marriage meant that under British law, Mohammed could only qualify for citizenship through her or her husband and not the biological father Bangoura, who lives in Leeds.
"I never received that letter. I am not stupid. I can't receive that letter and let my son go on holiday," said Keita, who is unable to travel to Belgium to be with her son because her passport is with the Home Office awaiting renewal of her residence permit in Britain.
The trip, which Mohamed has made before, was to give the boy a holiday while his mother continued to work at a fast food restaurant. The Conservative Immigration Minister Caroline Nokes has now become involved in the case.
Officials later emailed the frantic mother a copy when she contacted them after Mohamed was turned away from his flight.
A Home Office spokesperson said, "A letter was sent to Mohamed's mother in March advising her that her son's passport had been revoked. We understand that despite this Mohamed was taken out of the UK in July and last Sunday was unable to re-enter as he did not hold a valid passport.
"The Immigration Minister has asked officials to urgently look into all available options in this case to reunite Mohamed with his mother."
Keita said of the minister's involvement: "I really hope that this can reunite me with Mohamed,"
But immigration lawyers told the Telegraph that because Mohamed has never applied for Guinean citizenship, this could leave him stateless.
Jan Doerfel, a barrister specialising in UK immigration law and who is informally advising friends of Mohamed's family, said that if Mohamed was not British by birth he was likely to be stateless, but that a stateless child who has lived in Britain for more than five years is eligible to apply for British citizenship under the law.
Doerfel, who branded the situation "truly shocking" said: "Mohamed should be urgently granted re-entry into the UK so that his application can be considered while he is in the UK".
Questions have been raised by politicians as to why Mohammed, who was driven to Belgium, was allowed to travel to Brussels by the Home Office after they had revoked his passport.
Catherine Bearder, a Liberal Democrat MEP in Brussels, has been helping the family.
"This is symptomatic of the Home Office hostile environment," she said, "This poor lad is supposed to starting school in Sheffield next week. Instead he is stateless and stuck in a foreign country away from his mum."
"The British Embassy in Belgium need to issue him an emergency travel document immediately so this can be sorted out in the UK".
