Home is a prison.

No, quite honestly there isn't a lot you can do. It honestly is not your place. This is up to patient, family and their MD to discuss. You might suggest that you have witnessed other ways of handling diabetes and food intake and would be happy to discuss that if they should ever wish, but even at that simple kind statement families may take offense.
Might I ask if you have access to a Social Worker in this case? Because THAT person can approach this discussion, and it is in fact a part of their job.
Are you working with an agency? They, too, can guide you in this.
My best out to you. You've a very kind heart.

"Stealing" food from FAMILY? Who's banned him from Thanksgiving dinner? This is not borderline abuse, it's full fledged, flat out abuse! A caring family would have healthy, diabetic friendly food AVAILABLE to this person rather than trying to starve him "for his own good".

Report this dreadful family to APS that's my suggestion.

This seems to be very cruel and inhumane behavior on behalf of the family of your client. A person only gets one meal a day from Meals on Wheels, so I'm sure your client is very hungry, and Ramen is probably one the worst things a diabetic can eat, as the carbs turn to sugar. No wonder he is "stealing" food from his family as he's very hungry, and his body is probably craving something more nutritional. His family(and I use that term loosely)is not making sure that he's getting enough to eat and something needs to be done about that.
His environment is actually worse than a prison, as even prisoners get 3 meals a day
I would report this family(anonymously if you prefer)to APS, and tell them what you're witnessing. They will come out and investigate.
And then if you're not comfortable there, I would find a new client to care for as there are many elders needing assistance in one way or another.

All you can do is to speak to the family members who are in charge. If they want to talk to you about this issue they will. If not, there isn’t much you can do about it.

Have you considered looking for another job with a different client?

Best wishes to you.