Ok, what a kid is at 16 doesn't reflect on what a kid will be at 25 or that they are not a good kid. Real life example is my sister. All of high school she had blue, pink, green, violet, or any combo of the above. ALL of high school. she also had 39 holes in her head, most in her ears but still! What were her hobbies? She was leader of our church rock band... she was a straight A student with all honors classes... She liked to memorize Bible verses and read books on theology. So you look at the cover and say, "messed up kid" but if you KNEW her, "wow, she is just rebelling in her looks, and is actually a really good kid".
Also she is now 25 years old and her UNDERGRAD degree was in Biomedical Engineering... she is the TOP of her field and has different foreign countries begging her to do her research in their country. She doesn't dye her hair now and she has let the holes all heal up but she learned so much from those things.
She is one of the bravest people I know because of the BS she got for her looks in high school. She knows how to deal with it gracefully when people blow her off and they are wrong for it. It is part of the reason she is at the top of her field.
You can be embarrassed all you want, but I suggest you talk with him (and I do mean talk with not give orders about how you wish he would change). I bet the reason that he doesn't say much in front of you is because he already knows that you judge the book by the cover and sees no reason to fight you on it so is gracefully just keeping his mouth shut.