How to deal with an unhygienic human subject? The human subject that I really depend on is extremely unhygienic. Not simply sweaty way, but full-on stench, black teeth and dirty clothes.

I feel responsible for the lab equipment and space that I am going to use for my audio recording with him and I don't know what to do in such situation.

Anyone encountered such problems in their research? Is there any way professionally and politely ask someone to clean themselves up?

Any advice on how to deal with such a situation will be very appreciated.
Just put up with it.

It may be an unpleasant situation for you and others in the lab, but you haven't mentioned any significant negative consequences of his lack of hygiene; nor can I think of any.  I would just tolerate it.  If necessary, clean any dirty surfaces in the lab after each session.  You could also warn others to vacate the area when he is around, if it is harming others' work environment.

Your research is dependent on the subject's good will and participation.  So I would not do anything that risks his ongoing involvement.  Keep in mind that this is unlikely to be something that can be solved easily--this person is already causing unpleasantness to everyone else around him and yet persists in this behavior.