"if the referendum passes, what country will Sockatume be a citizen of? England or Scotland?"
No idea. It's one of the many things to be decided after the referendum and, presumably, independence in a years time or so if the vote is 50% + 1 for "yes".
"What passport will he have?"
No idea. See my previous response.
Independence means change, a lot of change over quite a short period of time. I expect a lot of handwaving and making shit up in a hurry about nationality, passports, currency, military affairs, border controls, representation in various supranational groups, embassies, tax offices, laws and judiciary, health and welfare, schooling, funding for assorted aspects of government, the whole nine yards. Usually independence is accomplished after a lot of bloodshed and burning when anything seems better than heads on pikes and clashing bands of armed marauders looting the countryside. Look at the mess the slaveholder's revolt in America in 1776 left in its wake after all.