A citizen is, in Aristotle's definition, someone who participates in deliberative and judicial office. The virtue of a citizen is therefore the virtue required to perform this function well — to deliberate soundly, to judge fairly, to command and be commanded appropriately. This will differ from one constitution to another, because different constitutions require different things from their citizens.
The virtue of a good man, by contrast, is a single thing: practical wisdom governing all the moral virtues in accordance with right reason. This does not change with the constitution. A courageous and just person is courageous and just whether they live in a democracy or a monarchy.
The specific mark of the citizen's virtue is the capacity to do both: to command well and to obey well. This is what distinguishes political rule from despotic rule. The despot commands without ever obeying; the slave obeys without ever commanding. The citizen does both, and the ability to do both is the mark of a free person who recognises others as their political equals.
In the best constitution — where rule is genuinely aimed at the common good and citizens are genuinely virtuous — the virtue of the citizen and the virtue of the man converge. The practically wise person who commands well also commands justly; the person who obeys well does so freely, not from servility but from recognition of legitimate authority.
This convergence is the political ideal, and its rarity is part of Aristotle's point. Most actual constitutions fall short of it, producing citizens who are competent without being good, or good without being genuinely political. The aspiration to close that gap is one of the deepest purposes of political life.
The relationship between the virtue of the good man and the virtue of the good citizen is examined in Book III, Chapters 4-5, as part of Aristotle's larger analysis of what citizenship and constitutional government require.