Where can I go for some specific dietary information?

Go to the John Hopkins web site. There is great information there.

Since this is sort of a big deal I would contact a oncologist.

Sometimes the phone is better than the internet.

^^That is sound advice by @johnpowell. A talk with a doctor or an RN working oncology would be most informative. I wish I had spent more time in oncology so I could give you a more authoritative response, Jeruba. But I didn’t and it was long ago.

Be careful when going to the Johns Hopkins website. There has been some malicious internet mimicry . Johns Hopkins is an excellent source of health information. Just be sure the url begins with “https://www.jhu.edu” and the grammar and spelling on the pages are impeccably correct.

What you’re talking about is called the chemotherapy diet in medical circles. It is a general recommendation of foods appropriate for the chemo patient, but it is a loose guideline because each person has different dietary requirements. The important thing is nutrition intake, period. For the chemo patient, gone are all the dietary restrictions recommended to the healthy patient. Often, getting them to eat something, anything is a major victory.

Here are some webpages concerning the chemotherapy diet from the Mayo Clinic, WebMD and other authoritative sources along with a book recommendation:

From Cancer Research UK: What should I eat during chemotherapy?

From Chemotherapy-dot-com: Eating Well During Chemotherapy

From WebMD; scroll down a bit to: Starting Chemotherapy: 15 Nutrition Tips

From the Mayo:
Sound nutritional advice when dealing with cancer

Tips for safe food choices during cancer treatment

A few treatments into the regime, patients often lose their apetites entirely. They are often nauseous and all foods taste terrible. This is a battle for the caregiver. And this is where a conversation with an RN who has been a primary caregiver for chemo patients would be invaluable to you. You find these nurses in oncology departments in hospitals, in oncology clinics, at homecare agencies, and at your local hospice.

More from the Mayo:

Eating during cancer treatment: Tips to make food tastier

No appetite? How to get nutrition during cancer treatment

Can taking ginger for nausea reduce or eliminate nausea and vomiting caused by chemotherapy?

Here are a selection of books on chemotherapy diets and homecare from Amazon (scroll down to the bottom to see a line of books). Rather than get involved in the crapshoot of choosing one, I would ask an oncology doctor or RN for a recommendation.

And here is a song for you, my friend, to brighten your night.

Thank you, and especially thank you, @Espiritus_Corvus, for all that information. I will look up all those resources.

My main question, about managing the transition from jail diet to home meals, may not even be relevant, then, is that it? because he may not be eating much anyway? I know that going from a restricted diet to a suddenly rich or full one (or going from famine to a quantity of ordinary food too fast) can be hazardous. But maybe that concern does not even apply here. Instead I guess the immediate problem is for him to eat at all while in treatment and in custody.

I think they will at least give him Ensure.

My experience with jail diets is that they are very high in carbohydrates because carbs are cheap. This is good news. All chemo diets across the board recommend a diet high in carbs.

As to transition to homecare, there should be no problem giving him solids, but his stomach may be shrunk from small meals, so at first he probably won’t eat the amounts he used to. Make food available to him at all times, and he will normalize.

There is also a thing in jail where sugary junk food is very popular. These foods are addicting. He may have a bit of a yen for these after he gets home. I would try to refrain from making these available to him. There are many carbs of much more higher quality than candy and cakes. Try to keep it healthy.

Ensure and it’s cheaper clones are valuable suppliments for chemo patients. Also, multivitamins and minerals. These are compact, quick nutritional deliveries for patients suffering from treatment-induced anorexia.

This is a small subset of your question, but thought it may be helpful to mention that most jails have a pre-approved menu that is reviewed by a nutritionist and followed. The menu is usually available somewhere, sometimes even online.

That may provide some lead in to what would be less of a shock, especially with protein selections.

I would ask their physician to refer to a registered dietician (not a nutritionist, if you’re in the USA the distinction is significant.) They can provide detailed menu plans and answer any questions you might have.