If he is a healthy weight and has no other signs, his fussiness doesn't likely indicate a medical issue.
An adult Shih tzu needs relatively few calories each day.
They sometimes skip meals or even go a day without eating and this is not an issue. If they have a lazy day or the weather is hot it's normal for them to skip meals.If they are getting any dental sticks, chews or treats this could be hugely curbing their appetite so this should stop if refusing meals.
Long term, we can't feed just mince as it isn't nutritionally balanced and Bertie will develop deficiencies.He needs calcium, iodine and certain vitamins which he wouldn't get in sufficient quantities from meat alone.
We must either feed a complete dog food (offer it at each meal, he will eat once hungry enough) or work with a canine nutritionist to start a home made diet plan. This may include mince, vegetables, ground bone or egg shells, grains, egg etc.
Please be aware that raw mince carries a risk of food poisoning and parasites and the risk is present to the dog and the humans in the home.