Yes, it may sound harsh but if that's the reality there's no point you or anyone else beating you up about it.
It's horrid when someone won't communicate with you, it can make you feel frustrated, despondent and downright angry. My h is a nightmare and he stonewalls or dismisses me a lot of the time. I feel the same as you in the 'gone off' sense but for different reasons. Communication is key to dealing with or sorting anything but it's easier said then done.
I know from my own personal experience, I can talk till I'm blue in the face but getting a response is a different matter. Sometimes my h makes all the right noises and nods in all the right places but I know it's either going in one ear and out the other or he's just not really listening.
The only alternative to this is writing it all down, either in a letter, email, text. Whichever you think he will read, it's not as easy to ignore the written word and he can read it over and over again rather than just hearing what he wants. Sadly some people have a 'head in the sand' stance on things, ignore it for long enough and it will go away :-/ not so, it just gets bigger till everything starts to piss you off.
Your dp needs to want to help himself, he needs to want to get fit and healthy and if he doesn't want to he's not going to do it. If it's not for himself then for his DC's sake in all honesty. They want daddy around for a long time to come whether you're together or not.
Is it possible he's depressed? issues with his job or anything like that? Or do you think he's just in a comfort zone and eating junk because he feels he's no one to impress anymore. Does he think, 'lovely dp gorgeous DC's what else could I possibly want'?
Is your dp happy? harsh as it may sound, he could just be plodding along as you seem to be doing. Sometimes we just fall out of love, we care for each other it doesn't feel like it's enough and something's missing.
It's your relationship and what you do is your business, if you feel there's hope then at least give him chance to make changes but I think he needs to know how this is really affecting you and how serious the situation as got. 