It's annoying this situation. Honestly, you just gotta be direct but honest - don't make up some excuse that doesn't hold up because he's gonna feel it and it can be even more awkward at work after. Just tell him you have a scheduling conflict or that you can't free yourself up in September, that's it. You can add something nice like "I would've liked to be there but it's not possible" so he doesn't think it's personal. Most people get that not everyone can come, even with an invitation. It's normal, we get invitations from coworkers that we can't/don't wanna honor, zero drama.
What's important is that you do it early, not like two weeks before. Contact him quick by message or face to face at work, tell him straight up, and then boom - you keep being cool with him at work. Don't drag it out or feel guilty about it, it's really not a big deal. The guy's gonna have tons of other guests, he's not gonna ruminate on your answer. People cancel weddings all the time for all sorts of reasons. Just be respectful telling him quick, and then we move on.