I'm assuming you have a restroom in your facility. Have you tried simply having him wash up in yours before you agree to treat him. Or perhaps some of the spray on anti-bacterial "soap" would help - just spray him yourself...
While we have to be culturally and socio-economically aware you are obviously not being direct enough. Tell him plainly to wash before the next treatment or you will refuse treatment, period. And actually do it. More than likely he won't, you'll have him leave, he'll come back weeks later and come in washed up for the appointment... There is absolutely nothing wrong with that.
While he is certainly not being abusive etc. an odor that bad could be derived from bad hygiene and diet but also bacteria, parasites, etc. which could be harmful to your other patients (and yourself), so I would consider it as the same level of seriousness as other problems in the clinic.