coworkers plural? and are these hand me downs or gifts (like secret Santa's) Hand me downs are easier to deal with " thanks Bob but I still have the 5 shirts you gave me last month my closet if full, you should pass these on to someone who needs them" IF they are actual gifts then there is nothing you can say except " thank you" , he can try to hint before the gift occasion.
Does he wear the ones he has already been given? He might have become "old man shirt" guy. If you wear a sparkly unicorn sweater to work twice a week , you may get sparkle unicorns for EVER from coworkers for EVERY occasion........Mary, I went to Hawaii I got everyone a lei except you sparkly unicorn for you , Hi all I'm back from Sweden chocolates for all Mary I had to look in 301 stores but here is a Swedish unicorn for you , have a baby Unicorn oneies , have surgery unicorn flower arrangement. If he has become "old man shirt "guy then he best thing to do is never were a "old man " shirt to work again, it will take time for people to stop but they will eventually (especially if he replaces old man shirts with some other distinctive thing)
Oh , you are not clear does he like the shirts? then it's likely a bad idea to try to change what he wears day to day.