What is the Sharīʿah ruling on a man who lives in a non-Muslim country and has stopped praying in the mosque, and now prays at home due to his discomfort with praying in the mosque because of shortcomings and transgressions on the part of the Muslims in that country?

This man is mistaken and acting contrary to the Sunnah by his actions. He is depriving himself of a great reward by missing the congregational prayer in the mosque, especially since he is in a non-Muslim country. He should, therefore, stick close to his brothers and be keen to be near them. The shortcoming should be remedied through wise counsel, not by abandoning the mosque. If he prays alone in his home, he is considered sinful by many scholars. So, he must be keen to attend congregational prayer in the mosque to avoid scholarly disagreement and not miss out on the reward.