Could be lots of things, I am afraid, including portion sizes.
The plums and the gravy, the yoghurt, the artificial sweeteners, too much protein, too little activity...
People often lose a significant amount of weight the first week or two, but then it slows down or stalls - often because the first weight loss was fluid, rather than fat. Carbs can often cause water retention.
And then there is the sheer calories.
Calories tend not to matter when you are very very low carbing, but the tipping point is different for different people. So it may well be that the plums, gravy, etc are taking him above the tipping point to where calories DO matter. Hope that makes sense.
Lots of us on here find that Low Carb doesn't automaticially mean weight loss, and we sometimes have to combine it with low cal, or lower cal too. Or intermittent fasting. Or something like that.
Give your husband a hug from me? I know exactly how demoralising it gets to see the weight not shifting, despite your every effort!