I think it's ok to question his delusions in a non-threatening way. You shouldn't just accept them as it will only reinforce them in his mind. But asking about them or questioning them is ok. You will know when to pull back or change the subject. You don't want to be cut out of his life. He may begin to see you as the enemy if you push too hard. But softly is ok. For example, if you can find a way to orchestrate it in a way for him to question his own delusion it may cause him to bring it up to his pdoc if only to prove you wrong

Good luck!