I sort of heard him say "mum" but it wasn't really very clear and it was one time. Now he constantly says Dada all day long. It's very clear and his favourite word. I'm a mother and am still waiting on the "mum, mom or mommy" :P It doesn't really bother me. Infact it is kind of funny because it's got my husband running to him all the time to help, while I take a well deserved coffee break. :P