First, welcome to PA!
Overall, I like Bobby. I think you've constructed him well, but I do have a couple questions. The average height for a 13 year old boy is roughly 5'. For Bobby to be that much larger at his young age suggests there is something else going on for him medically/genetically. He should still be growing during his teen years, in which case, he could be 7' or better by the time he's 17, and that makes him out of the norm for muggles and not by a little bit. Which suggest to me that there is some other genetic component going on for him? Not sure having him be 6' at age 13, as a muggle, is very realistic. Please revisit that piece and either make his height more reasonable for a muggle or explain the reason for his height. You'd also have to resolve how he is treated by his fellow classmates for being a foot taller than them, and, most likely, taller than most of the 7th years and much of the Hogwart staff as well.
Secondly, the other issue that seems to be not so workable for me is that if, at 6' tall, his arms hang down almost to his knees, that makes him seriously out of proportion and gives him a very ape-like appearance. I'm not sure what would make him so out of proportion to the rest of his own body, and you would also have to address the likelihood that he would be the butt of some horribly cruel jokes by a good portion of the student body.
One last thing, if he's 13, he would be a third year, unless he's been put back a year for some reason. Can you clarify that?