I don't think treating Grover's Disease with prednisone is a very good idea, especially since your husband has diabetes. Grover's is a relatively unimportant condition, which doesn't respond too well to most therapies, but which can often be controlled by topical creams and other anti-itch measures. Your husband may have to put up with less than complete control of his itch, but I suggest you consult your dermatologist about treatment options other than oral prednisone.
Take care.
Dr. Rockoff