There are two main factors that affect immune systems (well, three - but the last is genetics, and you can't influence that). Of the two that you can influence, one is good quality diet. Nutrition is the underpinner of health, so doing the best you can do there is important. Canidae is a good dry dog food, but it is still kibble (and kibble, by necessity of design, is rather light in meat content for a canine). So if you could add a bit of real food to his diet - be it raw or lightly cooked, that would be beneficial.
The second thing you can influence (a LOT) is stresses to the immune system. That's stress in general, but it is also things like vaccinations, de-worming, flea and tick treatments and so forth. Every one of those things is either live virus, or chemical insecticide. And whilst they can have their place and be useful - they can also do a lot of harm, especially to an immuno-suppressed individual. Certainly there are questions about whether a dog with immune system problems should receive vaccinations (which are intended for healthy animals only). And a lot of thought should be taken before assaulting the dog's immune system with pesticides also. Give only what is needed, based on specific risk (i.e. don't just throw on the flea repellent if you don't live in an area where fleas are actually a problem, or deworm the dog if he's not infected with worms) - and be sure not to give multiple treatments at the same time or close together. In this way, you can limit, if not largely eliminate, the stresses his immune system has to deal with, which in turn gives it more time to develop or recover.
