Originally Posted by
RichardInSF
The Keio plaza is right next to the Oedo subway line which takes you close to Tsukiji. If anything, I'd take the first subway and possibly a taxi from the closest station, Tsukijishijo.
The Oedo subway line runs very deep underground and the OP's father may not find the escalators easy to manage. Some stations also involve a lot of walk to get to the platform. Tsukiji-shijo station is only about 150m from the main entrance to the fish market.
A taxi from the Keio Plaza to the market will cost about 4100-4200 yen including late night/early morning surcharges and highway tolls and takes about 20 minutes.