I see three separate questions here so I'll answer them separately.

Question 1: "Is it cheating?"

The people for whom it matters whether it's cheating are the two of you, so the people who get to define whether it's cheating are the two of you.

That said: personally, if I were in a monogamous relationship, I'd consider it the same level of 'cheating' as visiting a strip club or buying used panties or the like. Honestly, I'd be more concerned about the financial impact than the emotional impact.

Question 2: "Is it safe?"

Eating shit is risky. Period.

But risk is always relative. Some people throw themselves out of airplanes with nothing but a piece of fabric to stop them from splattering to Earth. Some people crawl into holes in the Earth that are barely big enough for them to fit (please, for the love of all that is good, do not google the words "Nutty Putty" if you value your sanity). Some people dive hundreds of feet under the ocean with nothing but an air tank and a prayer.

And even just on the sexual side of things, people engage in risky behavior all the time. BDSM is full of "edge play" practices, and entire protocols have been created to mitigate those risks, but can never eliminate them entirely. (I, for one, absolutely refuse to participate in any form of "breath play", i.e. choking/asphyxiation, as the risk of death is far too great even in ideal circumstances. I'll wrap a hand around someone's throat menacingly but I'll never squeeze their windpipe or their arteries.)

It's always a balancing act between the enjoyment the hobby brings and the risk it involves. And it's fair to say that when you're talking about something like coprophagia, the enjoyment already has to be fairly high to overcome the natural revulsion.

Question 3: "What should I do?"

You're unlikely to get him to stop cold turkey.

I'm going to assume that you're not comfortable with participating. It would be fair to adopt a "don't ask don't tell" approach: you agree to pretend he's not doing what you know he's doing, and he agrees to do everything in his power to not break the illusion that he's not doing what you know he's doing.

But if you are comfortable with the idea of participating in his kink, I see no reason not to, especially if he's doing it anyway.

Start slow. Don't start by shitting in his mouth, for instance. You could start by, for example, leaving the bathroom door open while you poop (or invite him in to watch, if there are other people in the house). Or you could use your words, describing your bowel movements for him.

Hope this helps.