As Apricot says, be civil, even when it may pain you...
Besides, think about your mother. If she's happy, then you should be happy about this arguably...she's struggling with her partners terminal illness so the last thing she'll want is arguments, hassle et cetera. And, you may be on a perpetual guilt trip and feel really crap about yourself once your mothers partner has passed away, from having not been all that civil with him. For peace of mind, and your mothers too, remain civil with the man, and of course, with his medication, a possible irrationality could emanate....