Mr Strickland's body was found in his Citroen Saxo car on the Ballydrain Road on the outskirts of Comber on 11 January, 2012. He had been shot in the face at point blank range.
Ian Weir 29, and from Derryboye Road in Killinchy and his brother Jason have already pleaded guilty to the murder.
On Wednesday morning, shortly before 11am, Ian Weir entered courtroom 13 handcuffed to a prison officer.
As he passed the dock, he nodded to Stephen McCaughey, 26, of Shackelton Walk, Newtownards, but ignored his father, Jimmy Seales, 56 and from Ballykeel Road in Hillsborough.
McCaughey and Seales deny the murder.
The court heard that Ian Weir and his brother Jason had already pleaded guilty to the murder and on Wednesday morning, as a witness for the prosecution, he said he knew that there was ill-feeling between their father and Philip Strickland after comments had been written on Facebook.
Ian Weir said that he was in a yard in the Ballyglighorn Road in Comber on the 11 January 2012, along with his brother Jason and Stephen McCaughey.
He said that his father then arrived in his car. His brother was fighting with Philip Strickland in the yard and his father was holding a shotgun, the court heard.
"I couldn't see that at the time because it was dark," he added. "But he later threw it into my car and I pushed it into the floor of my Mercedes."
"Did you hear anything?" the prosecution QC asked.
"I heard a loud bang as Philip Strickland approached my father," he answered.
Ian Weir said that he didn't know what the loud bang was, but he said that Philip Strickland collapsed saying 'I'm sorry Jimmy' or something.
It is the Crown's case that after being shot in the leg, Mr Strickland was bundled into the boot of his own car and driven a short distance to the Ballydrain Road, where he was shot in the face.
On the Ballydrain Road, where Philip Strickland's body was found, Ian Weir said that there was another loud bang. The same one as he had heard in the yard.
He said that his father was standing at the driver's side of Philip Strickland's car.
"I got back into my car. My father walked towards me - he had a shotgun in his hand," he added.
Mr Weir claimed he then walked back to his car and was followed by his father, who got into the car and threw the shotgun at him, which he then pushed onto the floor.
He told the jury he left the scene with his father and drove back to their house in Killinchy.
When asked if he talked to his father about what had happened, Mr Weir said: "I think I said, what was all that about."
Mr Weir claimed his father told him that Mr Strickland had beaten him and urinated on him when he was lying on the ground.
Under cross-examination by the defence QC, Ian Weir admitted that he was an habitual cannabis user, but denied he was a dishonest person.
He admitted that he had told lies initially to protect his father.
"I love my father," he said. "I love him today and I will always love him."
At this point, Jimmy Seales wept in the dock.