I see Kevin McKidd is listed in IMDB credits...if it is accurate (This is a user edited site) then there are countless possibilities considering this is Trainspotting.
Going by the clips I've seen I think it could have something to do with a hallucination by spud.
I hope he is.
Love is the law...
http://www.digitalspy.com/movies/trainspotting/news/a808516/trainspotting-2-kevin-mckidds-tommy-may-be-dead-but-hell-still-appear/
Seems like it might just be archive footage.
Love is the law...
He appears in flashback when the boys go to memorialize him at the mountain from the first film, were Renton gives Tommy his *beep* being Scottish" speech.
"All those things I can do, all those powers. And I couldn't even save him."