Johnstone's Dad was on the books of Preston during the 1990's.
Dad Glenn is looking forward to his return to Deepdale, especially to see his son in action for England.
"I'm proud of him, I enjoy watching Sam play and we always have a chat after games about how things went," Johnstone Senior told theFA.com.
"He's been in the England set-up since the U16s and has gone from there and has been handled well by them.
"They gave him the experience of playing at the age group above a couple of years ago, before bringing him back.
"It's a long time since I played for Preston, but I've got some decent memories.
"The knee I injured in 1993 is absolutely gone now, so my involvement in football is limited to helping out a local side.
"I hadn't been to Deepdale for a long time until March when Sam and myself went to watch Preston's goalless draw against Scunthorpe."