I'd be inclined to let him wear it when he chooses to wear it (excepting, of course, state occasions like weddings and funerals). If that's where he went to summer camp, that's where he went to summer camp. And that's how I'd answer questions: "oh, it's his camp t-shirt". Wearing the shirt is not pretentious, nor is saying merely "it's a camp shirt." OTOH, having to use seven paragraphs to explain that he went to Special Super-Duper Snowflake Camp because he's just magically deliciously brilliant is unequivocally boorish (and we all know parents who'd do that).
