Boosting a dog's immune system is primarily about good quality diet and exercise, and removing stresses from his environment. You would not, for example, give vaccinations or elective surgeries (including spay/neuter) to a dog with a supressed immune system. Those are serious stresses, and counterproductive if what you're trying to do is boost or rebuild a compromised immune system.
On diet, raw or homecooked is probably best. But alternatively, a very high quality kibble and supplements if necessary/appropriate. I wouldn't personally give missing link to a dog that suffered certain allergies and/or frequent yeast infections - but it can be useful for most others.