The new press release for episode 3.23 (“Let’s Call the Whole Thing Off”) of “Brothers & Sisters” confirms that Luke will be appearing in the May 3 episode aswell:

“Let’s Call the Whole Thing Off” – When Kitty and Robert reach a crossroads in their marriage, she is forced to reflect on her father’s (guest starring Tom Skerritt as William Walker) past indiscretions and decide what’s most important in life. Meanwhile, Ryan makes a discovery about his mother’s death and takes a huge leap in the wrong direction to vie for Rebecca’s affection, on “Brothers & Sisters,” SUNDAY, MAY 3 (10:01-11:00 p.m., ET) on the ABC Television Network.

[...]  Guest starring are Tom Skerritt as William Walker, Luke Grimes as Ryan Lafferty, [...]. “Let’s Call the Whole Thing Off” was written by Peter Calloway and Daniel Silk and directed by Laura Innes.

Promotional photos for the season finale (episode 3.24 “Mexico”) have been released and unfortunately Luke is none  of them; but judging by the photos’ caption it sounds like he might be appearing:

BROTHERS & SISTERS – “Mexico” – When the Walker family reaches out to Tommy, they’re devastated to find that he may be unwilling to accept their help. Meanwhile, Robert and Kitty reach a crossroads in their marriage that they may not be able to successfully navigate; Justin makes plans for the future, with or without Rebecca; and Saul makes a shocking admission about Ryan Lafferty’s mother, on the season finale of “Brothers & Sisters,” SUNDAY, MAY 10 (10:01-11:00 p.m., ET) on the ABC Television Network.