all the drugs in the world will only mask symptoms... they are not cures... cures are problem fixers and the problem is a compromised immune system clearly by the fact he cannot fight off the staph that is around everywhere in our world and we are all subjected to it, but only those who are weak in immune system cannot fight it off naturally.
He is sensitive because he is on a diet that is not a diet he can manage.. he is masking symptoms by masking problems when you feed him a food like Pro Plan that is devoid of any real nutrition. Sorry, but Purina is the worst dog food maker bar none.
Not one Purina product should be fed to ANY dog... but thats my NOT SO humble opinion from years of experience and years of knowledge about the company.
I would never feed a dog a purina product ever...
but thats just me... he is sensitive because his stomach has never had an opportunity to process real food for any length of time. Runny poops are perfectly normal actually but we owners automatically think that runny means 'bad'... not so.... the normal digestive process is to acclimate slowly to a dietary change. so if you acclimate him slowly to a raw food change he will process it but it takes time.. often it takes months for a dogs system to finally run effectively on a food that is an optimal nutrition item..
beef is a great source of protein and a natural one that dogs in the wild would eat, so is lamb, and so is pork and so is goat and rabbit, chicken and turkey would be last... fish would also be one they would naturally find if they could hunt...
so My recommendation is to use a high value source of food that you add to his 'kibble' while you take a month or two to transition him to a totally new diet that has only real foods that YOU KNOW absolutely WHATS IN there...
If he likes turkey? great.. start with turkey necks and tails... giblets and hearts too... give him some par boiled turkey necks chopped into 1-2 inch segments. Par boiling means like 3 minutes tops... it just scalds the flesh to that its sort of cooked but not entirely. If you give him stew meat like lamb stew meat or beef stew meat same deal... either flash sear in a really hot pan with olive oil and sear the outer flesh for a minute or a minute and half and then cool it and chop it and serve it with his 'kibble' but remove the portion of kibble amount and add the raw/ par cooked portion of meat... add some chopped kale, some kefir or yogurt (1/8 cup or so) and some digestive enzymes (pet digestive enzymes) and some rice (1/2 cup) with that much LESS kibble... so if you put in one cup of meat, 1/2 cup of rice and 1/8 cup of kefir or yogurt, and then remove that much kibble and mix this together and give it to him as a morning meal... add the echineacea and golden seal capsules sprinkle them over the food or give them to him in a meat ball or a bite of yogurt and he should start to show some signs of improvement in a month...
ALL vitamins are human grade.. I would not give my dog anything that was not human grade and the kind of supplements I take are the 'good one's... Solgar is a good brand, so is Doctor's Best, and so is Udo's choice or Natures Way herbals...
Echineacea and Golden seal by Natures way is about $10 and Quercetin and Bromelain is about the same and he could go through a bottle per month easily..
these will make a big difference but it takes a month or so to notice it...
Its cheaper to COOK for your dog than it is to buy ANY decent kibble and Pro Plan is barely decent and its more expensive than to simply cook him rice and meat daily if he only eats 2 cups a day? he should eat one big meal and not two small ones, it seems like it would be the opposite but dogs have a stomach that was intended to gorge and then not eat for days... so a larger meal per day or every other day is actually better for than small meals per day. Their stomach must stretch out for them to secrete the digestive enzymes that dogs have in their stomachs.. if they never stretch the stomach? Guess what? NO digestive enzymes secret and therefore their food goes mostly un digested.... the complete reverse of how we humans try to eat...
try feeding him 4 cups of FOOD ( real food you have prepared) every other day... same amount of food completely different assimilation and completely different bio availability...
I feed mine raw and I feed large meals now 4-5 times a week... one or two days of fasting.... 3 would be optimal, but they simply annoy me on the days they do not eat because we are eating, not because they are 'hungry'.... they are not hungry (one weighs 84lbs and one weighs 94 lbs).... healthy happy and not hungry, just bored and used to having treats and snacks from us...
Just a suggestion...
what have you got to lose? Just try it for a few months and see what happens? improvement will be noticed in a month... and only optimal health will be achieved when he has been doing this for several months!