By my understanding that would make him your first cousin once removed [1]. He's either your dad or your mum's cousin, you're one generation away from them so he's a first cousin once removed.
[1] https://en.wikipedia.org/wiki/Cousin#First_cousins_once_remo...
By my understanding that would make him your first cousin once removed [1]. He's either your dad or your mum's cousin, you're one generation away from them so he's a first cousin once removed.
[1] https://en.wikipedia.org/wiki/Cousin#First_cousins_once_remo...