As real_ness mentioned, it's just a pilot. We don't know if it will get picked to series and if yes, when they would film the rest of the season.
If he booked it before knowing about the cancellation, he might only appear in the pilot or be a recurring character in a handful of episodes. Heck, he might get killed off in the first 5 minutes.
After Reign Edwards (Leanna) went to New Zealand to film "The Wilds", it took over a year for them to go back to NZ to film Season 1; despite the show being picked up before they even finished the pilot.
He filmed an entire movie during hiatus between S3 and S4; why not film a few episodes? He's too talented to sit at home and look pretty; waiting on CBS to correct their mistake