While in general people don't like to be asked to give up their seat, it can be worth having a honest conversation with the person in the window seat when you sit down and offer them the option to switch to the aisle (assuming your husband can safely sit in the window seat). Particularly because aisle vs middle is negligible for most people, not like window vs middle
If they say no, the flight attendant can often help you find alternative seats.
In fact, you may be able to speak to the gate agent about this and they may be able to help resolve this before you board.
Of course give JetBlue a call as well as mentioned above, but if the call center person doesn't help you out or tells you it needs to be done day of at the airport, what I previously listed are some options you can try day of.