this sounds gay, but for my style challenged close friends, i go shopping with them.
it's hard to tell what look would work for him...different body types, tapers, cuts and fits all make a difference. trial and error you learn what works for you.
I used to wear alot of long sleeve, dressy collared shirts. They'd look great, but my body type isn't flattering for them. Now i just wear really cool t shirts which taper my body well (dominant traps, shoulders and arms) most of the time with dressy jeans and nice shoes. I can get in pretty much any club in south beach with that look
if he likes wearing t shirts, just have him upgrade his t shirts. Get some vintage type t shirts that are cool but kinda dressy. san fran has a ton of botique type shops, he could find a little botique with good clothes if he really tried.
same with the sneakers...he just needs to wear more stylish sneakers. you can find some badass prada sneakers on sale online. lacoste has some really badass sneakers too
for people with no style whatsoever, i'd start him off with kenneth cole. kenneth cole has always been money. Spend a grand in there, and you'll come out a new man. Their shoes have always been great, but the rest of their clothes are really good.
You can find some good deals and cool stuff on bluefly.com too.
revolveclothing (someone from here put me onto them ) is a good base point for learning whats hot now. But alot of the shit on there is straight up fashion emergency, so gotta buy the right stuff.
invest in 2-3 pairs of good jeans....zara has amazing jeans for cheap, but if he wants something that'll last invest in a pair or two of AG, true religion or diesels. i even like Red Engine jeans, they're dope.