Some cats were never taught bathing as kittens, others are just lazy,
still others pick up substances they don't want to ingest (such as oily
dirt from a road or garage floor).

You might want to try using large moist cat bathing towels packaged as
"cat bath" to wipe him down occasionally. Start with just a gentle wipe
down his back and head when he's relaxed to get him accustomed to
them--otherwise you'll scare him and make him hostile toward them. Over
time he may let you wash more of him.

Before relying on these towels regularly, test the cat. Allow him just
enough time between cleanings to see if he starts cleaning himself. If
so, that means he's been picking up substances on his fur that he
refuses to ingest, and the cloths clean that away enabling him to start
cleaning himself.

Otherwise, he'll probably just never be a cleaner and you can start
toweling him down more often. Increase timing and duration of the towel
baths as he becomes used to them. Good luck!

IMHO  usually only if they don't feel well.  Have you had TED check him
for health problems?  MLB

The cats that I see (as a groomer) that do not clean themselves are almost
always too overweight to do so. They just can't.
If the cat is overweight, then have your vet design a weight reduction
program.

If the cat is totally fit and heatthy I might recommend a session with a
groomer who will not only get the cat clean and off to a good start, but you
can also arrange for a lesson on how to manage it yourself. Sometimes just
learning how to handle a cat and restrain him safely is all you need.