He also has a diaper rash that just won't quit, he's had it for two months now. I'm wondering if there's something I'm missing...allergy or something that's stressing his body.
How is he otherwise? Happy? Developmentally on track? Any other signs of an allergy (rashes, digestive upset, hives, etc)? If there are other concerns, I would take him to a doctor and see if there might be something else going on. But if he's otherwise happy and healthy apart from colds, I wouldn't worry too much. Some kids just have less effective immune systems for whatever reason. You could try probiotics and see if those help.
How was his delivery? Did you have antibiotics? I know there are lots of things that can wipe out gut bacteria for the little ones, maybe you need to get a good probiotic? I had abx with DD1 and she got sick quite a bit for the first year or so. It got a bit better at 8ish months when I introduced probiotics. DD1 also had terrible yeast that was so hard to get rid of. Is your LO's rash yeast?