I just did this the other day for the first time (after going North many times).
He has to cross the street towards City Hall, go left a bit, then cross back over Park Row, as if heading towards J&R Records -- but staying closer to the bridge.
The road which goes down closest to the bridge is I believe Frankfort -- there are signs "to the FDR". Take that one, it's a pretty straight shot downhill to the waterfront. It becomes Dover, then you hit the Seaport and the path. A little trafficky but doable.