I know two people who are well over that weight who have never been asked to get a 2nd seat, unless you DH has an unusual body type, he should be fine. Usually as long as you aren't "spilling" out of your seat and into the seat next to you, you'll be fine. Also the arm rest must be able to be all the way down (I think that's an FAA thing). Airlines do have seat bealt extenders too, so don't wory about fitting in the seat but not being able to buckle.
If you really want to, you could just go ahead and purchase an extra seat when you buy your tickets. You could leave the seat between you empty and then not only will you have the row to yourself and be a bit more comfortable, you won't have to worry about having to do it at the airport should it be necessary (though I doubt it would be).