I don't think he is his own grand pa. He's technically grand pa to his step daughter's son but because his step daughter's son is also his half brother this doesn't mean it works back up the family tree and he's his own grand pa as well. His grand pa(s) are still the two grand pa(s) he had before anyone got married. I may have this wrong though as it's all confusing a hell and I've had a beer.
https://en.wikipedia.org/wiki/I%27m_My_Own_Grandpa
And it's based off a Norwich Connecticut real-life occurrence. Messed up.