You sound like a loving dad to me too. Don't worry. Children go through phases of prefering one partner to another. My son went through a phase a little while ago of not wanting to be with his dad. It was tough for him but just being a dad brought him around.
Just keep being the loving father you are. He will know who his dad is and as time goes on will grow out of the mummy phase.
Kids like routine and if this is changing he will cling to what he knows....mummy.
Try not to show your hurt and just be the best dad you can be.
hope it works out,
nicki