D.B.
Hi C.! I moved across state lines a few times as a child - mostly during the summertime, but once in the middle of the year.
I would be more concerned with how his studies are going to match up than meeting friends. If your husband must move regardless of when the two of you move, then you will know what school district he is going to be enrolling in. I suggest calling the principals of both his current and future school and finding out a little about the curriculum before making your decision.
The reason I say this is because the one time I moved mid-year, I was drastically put off course when my classwork did not match up --- I felt constantly behind the other students when I was used to being one of the more gifted ones in my previous class. It is hard to make friends under those circumstances - when you don't feel you are making a good first impression --- and remember how quickly kids can turn cruel if you don't know something in class and also remember how quickly kids categorize each other.
Also because of the "group" issue, he may have a difficult time making good friends mid-year because kids at that age have already broken off into best friend pairs and cliques during the year and your step-son may get stuck having to hang around the few other kids left out by that process. I remember hanging around the one other "new kid" and a foreign exchange student who were both nice enough, but we had nothing in common really... The next year though, started fresh and as buddies and cliques reassembled, I was able to find a group of kids I really liked.
So, I guess based on my experience I would wait until summer. Usually during the summer, I would find other kids in the neighborhood to play with regardless of what was going on at school especially at age 12.
If the curriculums match up for an easy transition, it may be a different story. That just hasn't been my case - School curriculums can vary so much state to state. Even the distribution of students and grades may be different. Across the country, some 12 year olds are in gradeschool, some in middle school, some in junior high, some smaller schools stick them right in with the high schoolers... so this may be a volatile time of change for him and his classmates anyways -- I wouldn't want to rush him if you don't have to.
Take care, good luck, and Bon Voyage! Sounds like exciting times all around --- Congratulations!