Here is a site that lists all plants toxic to cats.It does not specifically list christmas cactus but the plant may actualy have another name. To be on the safe side I would NEVER allow him to chew any plants at all.
http://www.cfainc.org/articles/plants-non-toxic.html
Please let me know if you still have questions about this. I will be happy to help.
Feline Healthcare Expert
Expert in feline health and behavior. 20 years experience with cats.