Folate and vitamin B12 are the two vitamins that are most important for making blood cells, including white blood cells. Whether taking these vitamins helps your husband's white count would
You may know that some people also take iron supplements to boost their blood counts. A lack of iron isn't uncommon, but similar to folate and B12 deficiencies, it would be very unusual to have a low white blood cell count with a normal red blood cell count. In addition, iron can cause constipation, so I wouldn't recommend your husband take this supplement unless a doctor confirms he's low on iron.
Since you're concerned about your husband's white blood cell count, you and your husband should discuss the matter with his doctor. Try to find out just how low your husband's white blood cell count is, and ask his doctor if it's low enough to cause increased infections or other problems. It's also reasonable to ask the doctor if more should be done to find out the cause of his low white blood cell count. In general, if your husband has abnormal bloodwork that concerns him and/or you, his doctor should be able to explain a plan for either addressing it or following it up.
depend on whether he was low on these vitamins to begin with. Usually a lack of folate and/or vitamin B12 causes a low red blood cell count as well. If his red blood cell count isn't low, then it's less likely that his low white blood cell count is due to vitamin deficiency, so taking vitamins probably won't help. There's no harm, though, in taking these supplements.