In my experience, the only thing that would motivate him to go back to the JW's is a feeling of hopelessness and/or a lack of meaning or purpose in his life.
I think you need to try and discover (tactfully) what it is that he wants from life.
The Watchtower has nothing to offer him but false promises of a wonderful life in a perfect new world. This is the perfect escape from the real and actual world that we live in but it is nothing more than a blind alley with no end that will, ultimately, make him feel betrayed and deceived.
For a long time after I left I had occaisional 'urges' to return to the hall but after quite some years (eight or so) I came to realise that this was because I had not filled the 'void' that the JW's had left with something purposeful.
My guess is that your dear husband is in a similar situation and that he has a void in his life that has not been filled. Most people join a cult because they need a purpose and if you leave, that need for meaning and purpose does not magically disappear. And importantly, that sense of meaning is rarely to be found in human relationships alone so do not be offended if he needs something else in his life.
Granted, the JW's do offer a very attractive package but when one delves beneath the surface just a little it is exposed for what it is. This is something only he can do and as I say, in my experience, if one pressures a partner into being critical of the JW's it only serves to reinforce their claim that family members (such as you) will pursecute them aided by Satan. I know its mad and underhanded but that is how they work and in order to combat their tactics you need to play it carefully.
I do hope this is at least of some little help. I'm afraid I can only speak of my own experience and I do appreciate that every case is unique. Nevertheless, I do believe that the principles remain the same so I wish you well in your efforts and the very best for the future.
It would not surprise me if he has been feeling quite 'low' lately and that this is just a reaction to it but please do keep us informed.
Love & peace.