Joel Amade, 18, has started giving evidence at the Old Bailey in central London, where he is on trial for Jason's murder.
Amade admitted he was on bail for possession of a knife at the time of the killing and had been ordered to live at a care home in Ilford, east London, far from his home in Harrow on the other side of the capital.
The jury has been told the killing is believed to have been linked to friction between two gangs, one in Northolt and one in Harrow.
In November 2016 Jason, who lived in Northolt, travelled to King's Road in Harrow and took a selfie on Instagram in which he wrote: "Lurk round a K tryna leave mans head on the curb."
A screenshot of this same photograph from Jason's Instagram account was found on Amade's phone when he was arrested for an unrelated incident in March 2017.
On Wednesday, 3 April, Amade's barrister, Pavlos Panayi, QC, asked his client whether he knew Jason Isaacs.
"No," Amade replied.
"Can you explain how this screenshot was on your phone?" Mr Panayi asked.
"In 2016 an incident occurred in Kings Road and a friend of mine was stabbed. Everyone wanted to know who did it and a girl sent me this picture. She said it was possible he'd done it," Amade replied.
"Did you do anything with this image?" Mr Panayi asked.
"No," Amade replied.
"Why didn't you delete it?" Mr Panayi asked.
"I didn't care about it," Amade replied.
Mr Panayi then asked him about a video which was found on his phone in which he rapped about violence and drug dealing.
"Why were you singing about that?" Mr Panayi asked
"At the time I was representing gangs and the music was part of the culture. The video was glamorising gang culture," Amade replied.
Amade said he had been dealing in cannabis since the age of 16 but said the lyrics about violence were simply a reflection of the world in which he lived.
Mr Panayi ran through Amade's involvement with knife crime in the months running up to the murder.
He said that in July 2017 Amade was arrested with a knife in his tracksuit bottoms and in September 2017 he pleaded guilty to possessing an offensive weapon.
Amade was asked why he had a knife on that occasion.
"I saw someone stash the knife in a bag in a bush. I told a friend I would show it to him. I was going to take it to a friends and I got stopped on the way," Amade told the court.
Mr Panayi said that on 24 November 2017 — four days after Jason was stabbed — Amade was convicted of possessing a knife after an incident on 5 October 2017.
Mr Panayi asked Amade what happened on that occasion.
"I had been walking home from college when I was approached by two men. They ran up to me and stabbed me in the bicep….I ran away but they were getting closer so I pulled out a knife…and they ran away," Amade told the court.
Mr Panayi asked him why he had a knife on him that day.
"I got a phonecall that there were two people chasing people with knives and I thought I would arm myself," Amade told the court.
Mr Panayi then asked Amade to explain his movements on the day of the attack on Jason Isaacs.
He said he left the care home in Ilford and travelled by train and bus to Harrow, carrying a moped helmet with him.
The jury has seen CCTV of the attack on Jason Isaacs, in which four men on two mopeds chased him and his friends before stabbing him.
Amade said he brought his helmet with him because he intended to move his own moped and said it was not one of the vehicles involved in the murder.
He claimed he came to Harrow to attend a hip hop concert by his friend Kytel at Club KTM but did not end up going because it was restricted to people aged over 18 and he was only 17 at the time.
Amade said he then spent the rest of the evening with his friends before returning to the care home in Ilford around 2am. He denied he was anywhere near the vicinity of the murder scene.
Amade and his co-accused Khaleel Nyeila, also 18, deny murder.
The trial continues.