Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your fellow David has dark, crusty debris around the tip of his penis.
This is difficult to comment on without being able to see it and sample the debris to look at under the microscope and I suspect that's why no one has tackled this question.
In general I can say that this sort of debris indicates irritation and inflammation. This can be related to an infection, but it can also be caused by crystals in the urine, a primary urinary inflammatory condition called idiopathic interstitial cystitis or autoimmune disease (body attacks itself) that affects the mucous membranes or the junctions between haired skin and mucous membranes.
There are some things that you can do at home that may help.
Omega 3 fatty acids are natural anti-inflammatories that are important for healing and mucous membrane and skin health. I recommend an omega 3 fatty acid dose based upon the EPA portion (eicosapentanoic acid) of the supplement as if we do that the rest of the supplement will be properly balanced. Give him 20mg of EPA per pound of body weight per day. For example an 8 pound kitty could take 160mg of EPA per day.
Decreasing stress can help reduce inflammation as well.
Encouraging play behavior and exercise is a natural way to relieve stress. Fishing line toys for him to chase, hiding treats for him to "hunt down" in cat trees or kitty condos can be fun for him too. Perhaps building an outdoor enclosure on a patio or porch so she can go out safely would help since she enjoys her walks.
I recommend Feliway too as a stress reliever, especially if we believe that there are other pets in the home or outdoor animals that are causing him stress. This is a synthetic analog of a calming pheromone that cats use to mark things as safe or home.
You can also use a homeopathic called Rescue Remedy drops added to his food
or water along with Feliway.
Zylkene is another supplement that may help relieve his stress. Here is a link if you'd like to read more about it: http://vetoquinolusa.com/Products/Behavior/index.html
Increasing water consumption helps by diluting the urine, thus decreasing its irritant effects. You should feed him primarily canned food, and add chicken broth or warm water to increase fluid intake further. Kitty drinking fountains can be very beneficial to get him to drink more. He may play in the water, so I recommend putting it on a water proof matt. I know this is messy but play relieves stress too, so I would let him play in it.
If he continues to have excess debris around his penis then a veterinary visit is best to make sure that nothing more serious is going on.
Please let me know if you have any further questions.