"Strickland looked down at his boy. 'That's how you handle 'em, son. Don't give an inch, and maintain discipline at all times. Remember that word, son — discipline.' / His son nodded solemnly. He was a good boy, and he would learn the Strickland Code."
" Strickland reached down for his shotgun. 'I'm warnin' you—' / Tannen whipped out his pistol and shot the gun out of Strickland's hands. The shotgun flew into the bushes, half a dozen feet away. Strickland stared at Tannen. He had never seen anybody draw that fast! / Buford had turned his pistol on the boy. 'Drop it, sonny,' he ordered. / But Strickland's son still clutched his own shotgun. He looked over to his father, a silent question in his eyes. / What could Strickland do? Tannen had the drop on them. He didn't want to see his boy killed. / 'Do it, son,' Strickland said. / 'Yes, Pa.' His boy tossed the shotgun on to the ground. / 'Now, I'm warnin' you, Marshal,' Tannen continued in that same low-key voice. 'I'm here on a personal matter. And if you want to live to see your boy grow up, you just ride outta here for a few hours and leave me be!' / The rest of Tannen's gang had drawn their guns now, too. Marshal Strickland saw four pistols, all aimed at his chest. / He had no choice. He had to leave. He sighed and turned his horse around. / Tannen watched the horse walk away for a moment, then raised his gun and shot the marshal in the back. / Strickland fell from his horse, dead. / 'I lied, Marshal,' Tannen remarked amiably. He waved for his boys to follow him into town. They all knew they had some important killing to do. / The boy jumped from his horse and ran to his fallen father. "
After the Marshal dealt with Tannen, he taught his son the fundamental principle of "discipline" — a concept he would carry with him and teach his children and so forth, eventually resulting in the strict discipline exemplified by Edna and Stanford.
By 1983, Strickland's son Stanford was wearing his old clothes.
Behind the scenes
In the novelization and in a deleted scene, Roger witnesses his father being shot in the back by Tannen and runs to his fallen parent's side where, in the latter, the dying Marshal tells him "Remember, son. Discipline." However, in the novelization (see second Quote above), Strickland dies instantly.
While it is never made explicitly clear, he is presumably the father of Gerald and Edna Strickland.
The closing credits of Back to the Future Part III name Roger only as 'Strickland's Son'.