When I was on maternity leave, my son did the same thing. I attributed it to the fact that I was with him all day, Monday to Friday. Daddies provide a different type of loving and care than Mommies and that is a very good thing. You can find an article about this in the recent issue of Parenting magazine.
Don't have hurt feelings, encourage your son to build a strong bond with his daddy. I am in favor of having things that are just for them, my husband does bath time. It is their special ritual. Also, then you can have time to do something for yourself - like take a bubble bath, go to the store ALONE, read, talk on the phone to a friend, etc.
My husband also gets up early with my son on Saturdays and lets me sleep in while they go for a long walk and play in the playground of a local park. Robbie can't wait to go!
My son turned 17 months yesterday and now he differentiates between mommy and daddy depending on what he needs. It gets more interesting all the time:) C.