It's a small setback, but Sloane will be fine by next thursday, as long as he has no complications like collections/bowel obtruction
If he has surgery today, he'll probably be discharged by the end of the week at the latest (recovery for uncomplicated appendicitis is as little as 24 hours after surgery, though 48 hours is probably more typical).
I just hope it wasn't a badly perforated appendix (if he has really been symptomatic for a week, then that's a possibility).