Ideally, there should be no side effects, if that is the proper dose to control the Cushing's disease. Do you think Kurt is having issues? How long has he been taking the mitotane?
Kurt has been taking mitotane 3 weeks
Has your vet retested his adrenal glands with an ACTH stim test?
Here is a really excellent website on Cushing's disease and treatments. Be sure to look on the RIGHT side for more links. http://www.VeterinaryPartner.com/Content.plx?P=A&A=1193