Activating visa

Is it possible for a spouse to activate their immigrant visa (and set the process for receiving the green card in motion) while still in a transition period in terms of their residency? My husband must give three months' notice at his current job. If we're getting near the end of the 6 month window for activating the visa, but we've got plans in place for moving to the US (jobs etc) can he activate the visa even if he'll technically be residing in the UK for a month or two more?
Thanks!