Ms Belal says Shafqat Hussain was tortured for nine days before giving a confession, which he later withdrew. All other evidence against him was "purely circumstantial".
She says there has never been a credible investigation into his age or the validity of his torture claims.
The decision to postpone his execution on Tuesday is the fourth time he's been granted a last-minute reprieve. Ms Belal says she will continue to fight on her client's behalf until his execution is permanently halted.
"At the very least the government has to answer on what grounds they've got to proceed. They have to determine his age and produce and release evidence."
Shafqat Hussain claims he was electrocuted and had his fingernails pulled out
Cases like this are not uncommon, according to Maya Pastakia.
"Trials are often characterised by lack of access to fair legal counsel," said Ms Pastakia. "Often the accused in the initial stages will be given a state appointed lawyer who is often poorly trained and lacks competence.
"All judicial systems make mistakes and as long as the death penalty persists, innocent people will be executed. There is no going back."
How does Pakistan compare with the rest of the world?
Convicts are often flogged before execution in Iran
The issue of capital punishment sparks heated debates around the world. Although 99 nations have abolished the death penalty, 22 countries carried out executions in 2014.
China and North Korea are believed to be among the world's top executioners, however specific figures are difficult to obtain, because they are concealed by the authorities.
Amnesty International does collate figures from other countries, which show Iran and Saudi Arabia are responsible for the largest share of executions.
Iran confirmed 289 deaths last year, but 454 others were not officially acknowledged, Amnesty says. In Saudi Arabia at least 90 executions were carried out.
Pakistan is quickly catching up. Amnesty said on 10 June that 150 prisoners had been executed so far in 2015 - that compares to 90 in Saudi Arabia during roughly the same period. By the end of April the Iranian authorities had announced 130 executions, but 222 others were reported by other sources.
What is behind Pakistan's dramatic rise in executions? - BBC News