The CMs have said that he and Dr. Facilier are scheduled to maintain appearances at the Mark Twain indefinitely until ROA reopens.
As a DLR guy I'm so jealous of the special characters everyone gets at WDW. As far as rare DLR characters, for us it's probably been Bullseye, Hades, Roger Rabbit, and Mushu (all at seasonal or AP events). Prior to this year, Louis would have been on the list but as mom2rtk indicated he currently makes regular appearances on the Mark Twain Riverboat (in dry dock) due to the closure of Rivers of America because of SW Land construction.