I call Stuart's Dad by his first name. Even Stuart usually calls him that! He calls my parents by their first names as well. He hasn't spoke to his mother in close to 20 years. On the rare occasions she is brought up, he calls her by her first name, usually with some colorful adjectives around it.

He sort of has a step-mother. His Dad is divorced from his second wife too, but they are good friends and still keep in touch. We both call her by her first name too.

My Mom always called my grandma(my Dad's mom), Mom. It confused me to no end when I was a kid.