White patch on the tongue can be due to various conditions / diseases. If your husband is tobacco addict (smoking or chewing), it can be leukoplakia, which is a precancerous lesion. Try to scrape this patch with a toothbrush, if it doesn't go it can be any precancerous lesion or precancerous condition
(like leukoplakia, lichen planus
etc). If he doesn't have any oral habits but his oral hygiene is poor (dirty teeth, swollen gums, foul mouth odor), than it can be a fungal infection
called oral candidiasis
. If the patch is non-itchy, you can rule out the fungal infection. Moreover, it can be a feature of HIV infection: hairy leukoplakia
. This patch looks white with raised scaffoldings which appear like hair on it. Its an oral manifestation of AIDS and it occurs mostly on the side of tongue.
Complete clinical oral examination along with required tests is recommended. Its advised not to delay the treatment. A white patch in oral cavity for more than 2 months is something to worry about.