If he is in the USA in 1879, then see what the USA Census records say (there was a census in 1870 in USA). There were some state censuses in the mid-decennial period (1865, 1875)which might help you. The census usually tells you at least which country he was born in, if it was not the USA. If he was born in the USA, then look for his parents in the census (goes back to 1790 in the U.S., as I recall).
You could also make use of naturalisation records in the States: they can be quite informative.
Unlike in the UK, American newspapers often have obituaries of local personalities: worth searching. These obituaries often tell of the deceased's origins.
There is no obvious reason for someone to change their name from the uncommon one Pope to the equally uncommon Painter(Panter). If you want to hide, you assume a common surname instead; which is why I discount the fleeing story. Most Jacobite rebels fled to France if they wanted to get away. Remember, until the American Revolution (1775-1783) the States was still under British control.
Hope that helps.