According to Hollywood Life, it's being reported that Cody Walker, Paul's younger brother, was in talks to step into Fast & Furious 7:

The details of Cody possibly replacing Paul are interesting.  It seems it's more about using someone that looks like Paul, rather then a new character, but I guess they have to finish the movie, right? “[Producers] soon realized they needed someone who looked like Paul to finish the movie and that’s when they approached his nearly identical brother, Cody,” a source told Mail Online. “They can shoot Cody from behind and at distance. If it’s a shot they need Paul’s face in close up, they can CGI it later on.”

Universal Pictures has already said that they would work with Paul’s family to make sure that his character, Brian O’Connor, would have a fitting exit, so it makes sense that they would want to work with Cody. Cody, who normally lives in Oregon, has been staying with his family in Los Angeles while they grieve, and Daily Mail’s source says that Cody would only work on the film if it helped honor his late brother’s memory.

”If Cody agrees it’s because he wants to honor his brother’s memory,” the source continued. “There are many details that still need to be worked out, but right now the family and cast and crew are all still grieving.”