Most babies say dad or dada or something with "hard" sounds in it first - they are easier sounds for babies to consciously form when they first start to use speech intentionally, rather than just burbling and playing with sounds. Totally normal development, disheartening though it may be!
A lot of mums (myself included) also notice their babies say "mum mum mum" more when they're upset and grizzly - probably because it sounds a bit like a grumbly moan, and suits their mood!
It really doesn't mean that your bub prefers daddy, it's just an easier word for him to say on purpose at this stage. I think most of us have this problem - my advice is to make the most of it - say to hubby, "Kaleb wants you darling, I guess you'd better change his nappy!" tongue
This new forum is strange ...