First off, how does your partner want to handle it? With all due respect, you really have to find out his wishes. I know that it is YOUR family, but .... is it truly fair to out someone without their permission?
That being said, I am presuming that you and he have talked, and he is leaving the matter in your capable hands. Since you do have time, you might want to just casually start using 'he' vice 'she', when referring to him in conversation with your family. Sooner or later, they are going to get curious and ask about that, I suspect.
I think that it helps to keep explanations simple, at least at first. Basically, that he was born in the wrong body, and he is working to fix the problem, as best as he can [male mind and gender identity, but female body
] . I tend to use the reasoning that mad scientists have avoiding doing brain transplants, ever since Dr. Frankenstein got into so much trouble for it, so I am remodeling my body to make it more comfortable for me to live in. :P If they ask any questions, you can start making more elaborate explanations then.
When I came out, I told my youngest brother via email, as we lived so far apart. I told my parents via letter, after their trailer was safely snowed in, so they could not come up from Florida ( I was living in Michigan, at the time.), and start screaming at me. That way, if they started yelling at me over the phone, I could simply hang up, and unplug the phone. Actually, that first phone call went fairly well, and they didn't come up to Michigan that next summer, due to medical issues.
Overall, my coming out went fairly smoothly. Mind you, the next time that my dad saw me in person, he told me that I was going to die of AIDS, but, that is the way that he thought, too. He didn't understand, at all, as he was a very material person, the kind that thought that a tree was a tree, and not also a collection of molecules, with lots of empty space inbetween.
My mom was more understanding, and accepts me fully.
So far, the rest of my family is pretty much a tossup, looks like. I don't talk to them much, and we don't live in the same area, so I don't have to deal with them.