I would send him an email saying something like "hey friend. It doesn't matter if it's a fetish or not, or whether or not you have mutual interests, this is a person who doesn't respect your boundaries. It seems pretty clear to me that you have a sexual fetish for sneakers, and when you try to talk to me about them I feel violated or whatever word you want to use. It's boring and irritating. This reminds me of this post from NotAlwaysRight.
He and I have never been super close, but we ran in the same circles in school and I'd consider us solid acquaintances.
Sneaker fetishists discuss the shoes that turn them on (and off)
Particularly because I'm not single, am not remotely interested in him, and I've explicitly and repeatedly expressed my disinterest in the subject. He'll likely steer the conversation back to the wearing of bright red sneakers, or whatever, but I think that going there with him is the only reliable way to find out what's actually going on in this scenario. Then I sent her a brief email along the lines of, "Dear Coworker, I found some shoes that I thought might work with your outfit. A couple years ago, he sent me an IM to let me know that he had a coupon for Lady Foot Locker or some other athletic shoe retailer I don't remember and would I like to have it? How do I proceed? Actually, it seems like you've already tried that and he's seeing you having another conversation with him as being an opening.