The 'stomach'/belly isn't the most likely place you'll see them, so you may have misdiagnosed dirt or even black spots (the markings on the snake) as mites. The most likely place you'll see them are around the eyes and generally on the head.
Assuming you do have mites...
As Herpetology says, you need to get everything out of the enclosure, strip it down to a paper substrate (which you'll want to change often). I wouldn't use a hide in this situation, but it depends on various factors. If I was going to use a hide for a snake that size it would be something like a plastic yoghurt container with a hole cut out of the side (I'd be using something like that anyway), definitely not something with nooks, crannies or a textured surface.
You'll probably never get rid of mites with wood like in the picture. It's possible, but only with pretty extreme poisoning (it was relatively easy back in the day when we were using the old pest strips, but it's a long time since they were available, and they did have toxicity issues).
I haven't had a mite outbreak since the 1990s. At the time I had less than 10 snakes and it was a complete nightmare getting rid of them. I'd think they were gone then a few months later they'd pop up again because I'd missed a few and it was back to treating again. That experience inspired me to be extremely careful with new acquisitions and to always quarantine diligently. I've had them come in about three times since then but they've been spotted in quarantine and quarantine was effective (generally I do it at a separate address to reduce chance of spread).
The good news for you is that you only have one snake, which makes it much easier and there's no risk of transmitting them to any other reptiles. But 'having mites along his stomach' sounds like it's likely not actually mites anyway. Most mobile phones these days have cameras good enough to take macro shots of mites which will allow them to be identified. You might as well take pictures of the spots on the belly and share them, we'll be able to diagnose them for you.