Only one thing it can mean: he's a supporter of the Stuarts against the line of vulgar German hirelings who know occupy the English throne. '45 refers to the unsuccessful attempt in 1745 by Charles Edward Stuart, aka Bonnie Prince Charlie and The Young Pretender, to reclaim the throne for Charles' father James, aka the Old Pretender. (The Young Pretenders wouldn't be a bad name for a band, or perhaps an early-music consort.)
Not quite the same thing as Jacobins. Gotta watch your suffixes around here.