It has nothing to do with sexual orientation. It might have to do with the fact that the clothes are new and feel different, or because he was happy with the new clothes (at 15 the body is very hopeful and tosses up a boner even just because the kid is feeling cheerful) or it might be because he was thinking that someone he has a crush on will think he looks cute in his new stuff and his thoughts digressed to how cute that person is. But like MrDoctor said, boys at 15 are one giant hormone flow, meaning they get erections just whenever, often from things or in situations that aren't sexy. They can't do much about it besides live with it, so ignore it, he's probably trying to. You might not have noticed all the times he gets erections when wearing his *old* clothes.
Please try to relax about your kid's sexual orientation. He will figure it out sooner or later if he doesn't know yet, and after that maybe will even let you in on what it is if you have always been loving and supporting of the complete person he is.