He's a beautiful boy!!!!
My ball python did this, only she'd soak for hours and hours, in fact, once she got into her soaking bin, the only way she'd leave is if i physically took her out - which I'd do after 6 or 8 hours. I couldn't find mites on her either. In the end, it turns out she's about to shed, which I know you don't feel is the case with your guy.
The only other thing that comes to mind is to wonder if he may be constipated, when did he have his last bowel movement? I don't know if soaking would relieve this condition, but failing any other possibilities, might it be worth considering?