My son (similar age) calls both me and his mum "Daddy". He also has a habit of calling all animals Cows or Sheep. Like Erica says in the comment, it's pretty standard. They've learnt a single word which at the moment means "Parent/Adult/Someone that's not Me". As they learn more words they can elaborate on the distinction. Just reinforce the difference patiently ("No, I'm your daddy! That's your mommy", while smiling) and he'll get there eventually!
The best example I've ever seen was my nephew, who had learnt that his Grandparents' Dog's name was Pippa. He then called every dog Pippa. When he was told that a particular dog was not Pippa ("That's not Pippa, that's Rufus!", for instance), he would then call the other dog "Not-Pippa".