Can Vitamin-C Be Given To Boost The Immunity Against Respiratory Infections?

I ve seen the effects of high doses of Vitamin C can have on the bodys immune system particulary for respiratory problems, so if we were to do this for my elderly father who has just been diagnosed with Bullous Pemphigoid, would that help his body fight infections in general ( ie against colds, flu or even pneumonia,) even though hes on prednisone? We realise it won t help his diagnosis but just to keep him as healthy as possible while his immune system is suppressed. What else could you recommend we could do to help give him some quality of life? Natual remedies or not. At present his condition is controlled with only antibiotic ointment and 20mg of prednisone. Is this condition likely get worse? He is a diabetic.Thank you for your time.