Come to think of it, I remember he was a dashing man — I wonder what he's up to.
Literal
Come-to-think-of-it, dashing man was memory [subject-が] exists but how-doing [wonder-んだろな]?
鯔背 (いなせ, dashing/gallant) is a rare, evocative word — originally describing the fashionable young men of Edo's fish market district. そう言えば (come to think of it) triggers a memory. ~んだろうな trails off in wondering, with な as a masculine-reflective ending.