Malnutrition

You should look at the diet of the Inuit and Eskimos, peoples in that part of the world.

Does he eat organ meats, too? Some nutrients are better sourced from organs rather than muscle. You can look up what the nutritional content of meats and organs are and see what vitamins or minerals they're missing and what the effects of being low in those things are, as well as what timetable they kick in on.

Without carbs, he'll be forced to get his calories from fats, which is likely to be abundant in whatever meat he is eating. Lethargy, light-headedness, and nausea are very common in a person whose body is theoretically adapting to that difference source of energy. Anecdotally this could last from a day or two to a week or two. Your MC would be wise to scavenge for starchy roots or plants to alleviate some of these symptoms. Depending on how much hardship you want to throw at your character--some people experience uncontrollable diarrhea from sudden changes to a high-fat diet, rather than constipation.

Personally, I was on a stewed-meat-and-veggies diet for about one week a few years ago and felt very ill, to the point that I freaked out and got a pregnancy test (even though I was fairly confident I wasn't pregnant) because my symptoms were eerily similar to both of my horrible "morning" sickness experiences. I'm guessing it was a low blood sugar thing as I have a history of being on the hypoglycemic side of things. I also went down from about 128 lbs to 115, which I understand to be a terribly unhealthy amount to lose in the space of a week. Despite returning to my normal diet immediately, it took me about a year to gain that weight back.

Try googling "paleo diet" or "Whole30" or "ketogenic diets" as those are very protein-centric, low/no carb diets (that do include vegetables, ftr, so not quite a straight-across comparison, but similar). You might find some more stories detailing the symptoms of people who were/are on them.

Humans do quite well as carnivores. There would be no vitamin deficiencies, and, unless there is some protein intolerance or problem digesting fats, then it would be completely healthy.

It would depend on his personal biology. The vast majority of people are omnivores and could adapt for a time to a strict carnivore or vegetarian diet but depending upon their ancestry and the source of their foods will at some point have issues with any restricted diet. There are very small subsets of people that are carnivores and very small subsets that are vegetarian and the sources of those meats and vegetables will matter to them. There is no universal diet that is good for everyone. The Inuit as were mentioned above have been studied and are probably the closest to being carnivores, at least for about 9 months of the year, however their biology requires mostly a sea based diet. Give them equivalent volumes of beef and they tend to have terrible cholesterol issues. Take the poorer areas in Asia where for over 100 generations they've adapted to (meaning those who didn't tolerate it died out long ago) a soy based vegetarian diet. That works for them but but take someone from a different ethnicity that did not evolve eating soy, particularly a male and put them on the same diet, even if they were a vegetarian, they'll have health issues most likely.

Vitamin K is low to absent in meat, so issues with blood not clotting easily may be an issue.
Vitamin A likewise so there may be some blurring of vision, mild reduction in disease resistance same with E
Vitamin C deficiency is well documented in history and is called Scurvy
Vitamin D deficiency can lead to anything from Osteomalacia to osteoporosis, some people may be able to manage with enough sunlight, so weather plays a role.

We don't know your character's ethnicity or whether the new diet is something his ancestors evolved eating. We don't have enough information. Maybe we don't need that information? Maybe you want to pick and choose believable problems and don't supply the character's history such that nobody can really complain?

Constipation may be a significant issue as well as flatulence

King Neptune said:

Humans do quite well as carnivores. There would be no vitamin deficiencies, and, unless there is some protein intolerance or problem digesting fats, then it would be completely healthy.

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That depends on the person and what meat constitutes their diet. If their ancestors evolved with whatever that source of meat is then yes, for the most part if you are talking months to maybe a few years.With the exceptions of the above issues.

What are you trying to do to the character, and how bad do you want him to suffer?

Do you want permanent neurological damage, tooth loss, or what?

How is he/she getting this "mostly meat" diet?

It takes a very SMALL amount of carbohydrates to keep you out of ketosis. It's like the "wick" for burning the fat calories.

The standard recipe for pemmican is dried lean meat, pounded to a powder, mixed well with pounded dried berries, about 5-10% berries (measured by volume, a scoop of berries and 10-20 scoops of meat). Then you blend it into the kettle of melted fat and pour that mix into the molds. Disgusting stuff, but shaved off and made into soup it keeps you going.

So having him with "starch hunger" would be reasonable. And scavenging for green weeds.

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