LOL, dads can be like that. Just tell him to wear whatever he feels comfortable in. He's not going to stick out much, especially in summer. There are plenty of tourists, as well as tall blonde people who wear white athletic shoes and shorts and happen to be Bulgarian too. I've worked with people like that over there. Well, when he speaks English, people will surely notice he's a foreigner, but even then the reactions will be either neutral or positive. If you choose to visit a small non-touristy place you'll probably attract attention, but that's because people living in such places know each other and are always curious about visitors, regardless of their appearance or nationality. Anyway, he has nothing to worry about.