Take him to the vets. If the vet can't solve it, then by all means try
Feliway. The plug-in diffuser is the best. Jeffers Pet has the lowest
online price I've seen.

Cats will rub their face on objects to mark areas as a 'safe zone'. Feliway
mimics the facial pheromes. It's a subtle effect. I notice I need a refill
when my two boys get too rough playing or seem testy with one another.

If this is an inside only cat then the dirty coat appearance would be of
concern. A lot of times when I've seen 'greasy' or dirty looking cats there
has been a health problem. The excessive grooming could be an indication of
this as well.

I have seen cats over groom and worse because of food allergies. One of my
recent fosters didn't do well on most dry foods because of the high grain
content. Is the cat on a prescription diet? If so you don't want to change
it without consulting the vet.

One of the first things that let us know one of our cats was sick was a
greasy appearing coat.

I think I'd discuss your concerns with your vet who can check the cat over
and see if there is something brewing.

W