First, have you taken him to a vet for the tooth/gum problems? Dry food shouldn't make his gums bleed, so I strongly suspect an underlying issue.
You can shampoo cats, however, most I've known are, unappreciative, of the effort. lol (My sister had a Siamese that used to beg for his weekly bath, though, so...) However, most domestic felines avoid water because their skin can dry out and become very itchy when the oils and saliva are washed off too often.
Here's a post to some websites about animal food that you might want to read. The kibble/dry food you've been feeding him for years may be much less nutritionally sound than the bag implies: links on pet food.