Fear of aliens is indicative of a psychiatric problem that he should really get help for. I know you say he refuses to go to therapy - honestly he probably needs to see a psychiatrist for meds more than therapy. Think of how irrational it is to not turn the lights out because then the aliens will get you... if there are super-technologically-advanced aliens out there I don't think the 60w incandescent light bulb will stop them... I would consider you try to continually (gently) press him to see a psychiatrist without being overly confrontation and pushing him away.