My first suspicion is yard chemicals, yours or the neighbors. Mosquito Spraying. Residual chemicals. Carpet cleaners, insecticides.
Dietary, In order for his body to fight back, his body needs the tools to do it. I imagine you vaccinate, the ingredient in vaccination over a dogs life, the adjuvants do not leave the body. Heavy metals especially aluminum, add in fluoridated water his body cannot fight an infection. His thyroid activity may be diminished.
Real meat and fats. I probably would cook meat lightly. In bone broth to power his gut. Good meat with fats. Maybe add 1/4 cup of pumpkin to help his elimination stay normal. Empty his anal glands, no intervention needed. Because of the staph, raw meat would not be ideal. In a crock pot. Bone broth, sea salt, pot roast, or some other cut. A few carrots for vitamin A. Or some chicken or duck. Then you do not have to cook every day.
When you feed him, to his food add 2 tbsp of non GMO lecithin. This will boost his overall health and keep his liver in tip top shape. Every third day, with 1/4 tsp of cilantro to help remove heavy metals gently. 1 canned sardine in water for fish oil. 500 mg MSM for sulfur and diminish his symptoms. Add a scoop of liver powder or brewers yeast, this will improve his amounts of b vitamins, needed in each step of health. In an older dog, just like people, digestion is less than optimal, nutrients from food need some help to be absorbed and utilized.
Raw meaty bone to chew every other day. Cleans his teeth, and further improves nutrition.
To really make an impact on his health, add Teds alkalizing drink. 2 tbsp apple cider vinegar, add 1/2 tsp baking soda, let fizz. Add 1/2 glass of water. Divide this into 2 doses. Pour one over morning meal. One over evening.
This will do the work of alkalizing, improve circulation, sleep, clean up his digestive tract. A miracle in a glass.
Defluotidate his life, spring water to drink. To remove it from his body you can add Teds 20 Mule Team Borax remedy. This will improve his ability to heal. To make it, 1 liter of spring water add 1/8 tsp of Borax. Add 1 tsp to his water when you change it.
Now to kill the infection, 1 drop of lugols iodine on his feet, rub in. Alternate feet each day. This will wake up his thyroid. You could even wash them with a few drops of iodine in water. To kill the infection. If he is jowly this solution to wipe his lips and mouth
The internal remedy,
In a 1 liter bottle add spring water. Leave an inch or two at the top. Buy l lysine, l threonine, sodium ascorbate powders on amazon
A human adult dose for 130 lb person is this remedy. If he is a big guy, this will work for him. You are making 4 doses a day for 3 days. In the am, every hour, syringe in one dose. For 4 hours.
In your bottle, 500mg sodium ascorbate, 6000mg l lysine, 1000mg of l threonine. Shake, cap, store in frig. Mark the bottle with 12 equal marks.
Topically. Teds mite spray. Often.
I also soak our dogs in Epsom, they need lots of magnesium. So in a kiddy pool or the tub, ankle high water. 1 cup Epsom 1/2 cup if borax, have dog stand in it as much as you can.
You may repeat the lysine remedy another 3 days. But I think this will do it.
Wash his spots and areas and bedding in borax and Epsom . No softeners or scented products. Spray with mite spray. I spray our yard with a gallon sprayer 1 x a month in warm months. Detoxifying and kills offenders. Or diatomaceous earth. Mix with some soil in a spreader. Renew after rains.