Spiffy had the right answer, call social services and explain the situation, you could even lie a little and say he has talked of suicide and you haven't seen/heard him for a couple of days, that will get someone out there.
Police/social worker are much better equipped/trained for something like this.
If you are not sure of his mental state, a direct confrontation may not end well.
if he is mentally stable and just has bad hygiene, maybe suggesting a maid service would be received better?
on a less serious note:
my top three stinkiest encounters have been;
#3 the Indian guy who runs the local small liquor store, between the stench of BO and incense, ya damn near have to hold your breath if you go in.
#2 Vietnamese who live on kimchi and garlic. when they sweat that shit out of their pores, it can clear a movie theater.
#1 the top stinkiest encounter is without doubt when I worked at natural food color/flavor maker. We dealt with many companies that are kosher; Hunt-Wesson for example, and we were subject to bi-annual inspections by a couple of rabbi's.
these guys were your hard core Hassidic Jews, long curls, beads, and everything they wore was wool. I guess it was some religious thing, but they obviously didn't bath for looong stretches of time and would stink the place out like you wouldn't believe. I'm talking a production facility/warehouse of about 12,000 square feet totally engulfed in acrid rabbi stink!!!
We fucking hated those visits, not only did they stink, they were the most arrogant mofo's I have ever met in my life.
there was the crazy lady that used to come in to one of the Sav-On stores I managed that the cosmetic clerk would follow around they store with a can of air freshener spraying it, but what the hell, that nutty lady was the sister of the Queen of England and G.W. Bush seniors old girlfriend and she had the cracker-jack ring to prove it....LOL