A lot of guys think things that we might think are gross...not to be so gross on a girl. Plus, girls usually (not always) make their feet look so pretty, I suppose. Also, some people are just drawn to certain parts. Your guy might not really just like feet. I bet it's mostly because he's really into you...and you just also happen to have pretty feet. You said he tickles your feet, so maybe he also likes to make you laugh. Every person has his/her own reason for liking what he/she likes. Sorry, I could not give you a definite answer for this one (especially since I'm not a guy). :)
When I'm with a girl I like I tend to play with her feet sometimes. I started to play with them because girls feet are ticklish and it makes her smile and laugh. Also I think some girls have nice feet and with them painted and everything they look cute.