AP Lee Song is actually not that bad of a support when you think about it. Great peels with your ult, and you can also utilize your ulti to make ward-hop/insec plays. Your w scales with AP so building AP will help with getting bigger shields. Late game, your shield is extremely huge, and you can stack your face of the mountain active with your w to make your AD harder to take down.

The CDR in this build is mainly just to almost always have your ultimate up as it is the main source of peeling for your adc/ making plays.

You also take distortion boots so that you can R+Flash or make 'flashy' plays Kappa

Your Q procs abyssal scepter, which slows your opponents, and it also goes well with your e. Great item to catch people out and help your allies land skillshots.

Zekes gives you cdr, and has good synergy with most adcs who utilize crits.

Rabadons helps you get that extra AP from its passive, which makes your shields bigger.