Well, it's boots not bras. I'd ignore it. It's hard and you worry. Worst case scenario, he enjoys woman's clothing. And that's not all that bad. I'd just keep your availability there for chatting with him. Kids of this age start to stop talking to mom. So, you have to be around a lot and be available for the moments they do. Never make him feel judged and let him know he is safe to tell you anything and you will always accept him. He may open up to you. Or he may not be concerned about this and not know why he needs to tell you. In which case, let it be. Parenting is hard!
It's just boots. Yeah, you should ask him what is it about women's boots that he likes? Get to know your son.
He might just like feminine clothing, he could be trans, you should just talk to him about it but don’t be too pushy.