Does he have foot fetish
That's not what a fetish is. It would be a fetish if Kang can't get turned on by anything other than feet and was obsessed with them. He's being a good lover and exploring Yiyoung erogenous zones (which aren't just the genitals and nipples). Feet can be notoriously sensitive. I know this from experience (embarrassing incident).
OMG SHET ETO NA NGA SINASABI KO EH