You know, I think people end up doing what they want. And if your BF insists on wearing these shirts, I suspect it's that he actually likes them, or at least not having to buy other shirts.
If this is a huge turnoff, perhaps you need to consider another BF. However, before you go that far, how about *you* buying him a good number of shirts that you would like him to wear? Tell him that it'll hurt *your* feelings if he doesn't wear them.
The results should tell you a lot about his style preferences, as well as how he will treat you compared to others, in future.