MR

I’ve seen a few clips recently of Styx live, and he’s been playing on stage with the guys. Is he just a “guest” member only on “The mission” tracks or is he doing the full show?

Will is listed as "additional personnel" in The Mission credits. He was a major contributor to The Mission as he co-wrote almost every song with Tommy, played some instruments and produced the album. He is hired help I'm fairly certain he only played certain tracks during The Mission live shows. They don't need him for the entire catalogue.

He was the vocal producer for Regeneration I and II, and was part of Tommy's The Great Divide album (co-producer, engineer, backing vocals and guitar on some songs)

Oh.. and if you didn't know, Tommy and Will actually prepared most of the album in secret, without telling anyone else in Styx.

"We kinda snuck around for a long time. ...The idea of me coming to everybody else and saying, 'Look, Will and I have an idea for us to do an album about a mission to Mars' wasn't something I could confidently go in and talk about. I think everybody might have been overhearing little playbacks and this and that and the other and they were getting curious. I'm sure there were some discussions on the bus when I wasn't around. There finally came a point where it was, 'Let's bring everybody in.'"

https://ultimateclassicrock.com/styx-mission-new-album/

I'll weigh in on this, since I was at both of the Long Island "Mission Shows", last November.

Will came out, with no fanfare/introduction, & stood towards the back for ONLY "The Mission" portion of the show. When the band returned to play the hits, he was not present. I do believe, though, he rejoined them for the curtain call, once again with no fanfare/introduction.

Having no seen him present in any pictures/video from any other shows, I find it safe to say that he only joins the show for the Mission performances, to augment the sound.