Azmat Begg told BBC radio that his son Moazzam, who alleges that he has been tortured by his US handlers, was being driven insane by his detention in solitary confinement.
"From what I gather from different sources, it looks that he is deteriorating very badly and things are going badly physically and mentally," Begg said.
"I don't know how a person can stay in solitary confinement for such a long time and remain normal.
"Why do they want him to go mental? Is it because he has seen a lot of cruelty and a lot of irregularities and violations of human rights? That is possibly why he is kept aside, so he doesn't talk about what he has seen to the other prisoners," Begg told the British broadcaster.
Unfit for trial
He said that by the time his son's case reached the courts he would be unfit to defend himself. "By that time, mentally he will be finished. He won't be able to say anything. He will be a cabbage," Begg said.
Moazzam Begg, 36, was arrested in Pakistan in February 2002 and was among nine Britons known to have been detained at Guantanamo Bay.
Moazzam says he was tortured
by his captors
Four of five Britons released in March, and subsequently freed upon return home, are suing US Defence Secretary Donald Rumsfeld and other top US officials for alleged torture and abuse.
A report released this weekend revealed that another Briton still detained at Guantanamo has claimed he was subjected to abuse and humiliation.
Martin Mubanga, 31, told a visiting Foreign Office official he was kept shackled for so long that he urinated on himself, and then was forced to clean up the mess, that an interrogator stood on his hair and that he was subjected to extremely hot temperatures.
The Red Cross, in a report leaked in late November to the New York Times, said prisoner abuse at Guantanamo amounted to "a form of torture".
In a visit in June, the organisation witnessed a system devised to break the will of prisoners through "humiliating acts, solitary confinement, temperature extremes, use of forced positions".