Is your husband taking the kids to Macca's as the sole purpose of the journey, or is he dropping in to Macca's as part of the day out with the kids?
If it is the former, i.e. going out with the explicit goal to get fast food and taking the kids (leaving you at home?), then that indicates to me that he is comfortable with a non-threatening or non-judging presence which makes him feel free to indulge his fast food. He is surrounding himself with approving people, i.e. the kids, and avoiding conflict.
If it is the latter, i.e. stopping in to Macca's after a day out with the kids, then that can be seen as inconsequential. If I'm out on the road, I'd more than likely drop into Hungry Jacks, or KFC. There is nothing wrong with that, and I too am working to lose some abdominal fat, but I am not consumed by that goal at the expense of my mental health.
If he is sneaking the occasional fast food, then that's fine. If he is hitting the counter at your local Macca's several times a week, then to me that is an indication of either an addiction, or he could be punishing himself for not sticking to his training to lose the weight. I've seen the latter happen to people around me, and they've used different kinds of self-punishment from smoking, drugs, binge eating, and violent behaviour, when they were trying to kick the habit of smoking, drugs, binge eating, and violent behaviour.
The mind will always go back to whatever it did in the past to feel happy. It doesn't matter to the mind if that is a good activity or a bad activity; the mind will always go back to it until you discover why you were compelled to do that activity in the first place. This is a heavy sh*t, deep personal discovery you have to undertake to get to the root cause of why you do things. Your husband can benefit, and indeed every one else in this world, by reflecting on why you "did it" in the first place.
I need more information on what's going on in his mind to try and home in on the root cause.