Hi.. Thanks for the query.. Well, at such an elderly age with medical history which he already has, Thrush can occur due to poor immunity and reduced resistance of the body to diseases.. So it seems to be the most probable cause.. As yeast is an opportunistic organism and overgrows in individuals who has poor immunity therefore it can be a cause for him to get Oral Thrush.. Do not worry with proper maintenance of oral hygiene, gargling with Nystatin mouthwash and using Candid mouth paint will resolve the symptoms.. If he can eat then probiotics like Yogurt and garlic can help.. Hope this helps.. Regards..