Hi welcome to HCM
I have gone thru your query regarding putrid taste in mouth and difficulty in swallowing and I can understand your concern .
It is an appreciable fact that even at age of ninety he is enjoying sound mental and physical health .
What ever problems he is suffering are outcome of fauly life style and food habits .
Iwould like to suggest to locate the cause and remove , it will give relief to some extent .
Difficulty swallowing (dysphagia) may occur at any age but is more common in the elderly.
There can be many factors causing putrid taste . Individuals suffering from digestive system diseases like Acid Reflux, & In take of too much of medication may also result in bitter sensation .
Suffering from digestive system diseases like Acid Reflux,. Ulcers arising
in the stomach upwards through the esophagus, connects , the mouth with the stomach. As a result , toxins accumulated in our system due to faulty intake of food , cause you putrid taste and difficult breathing .

. Any condition that weakens or damages the muscles and nerves used for swallowing may cause dysphagia. . Difficulty chewing food because of poor teeth or ill fitting dentures may cause dysphagia. An infection is another cause. Narrowing of the esophagus caused by a stricture or a tumor of the head, neck or chest, may also create swallowing problems.

Detoxing of system -by Ayurvedic Kunjal - Drink 5-6 glasses of warm water empty stomach , hold for 2-3 minutes. Vomit all the water . Clean all the cavities -eyes ,nose , mouth by sniffing , coughing ,gargling . Repeat this twice a day for 3 days every fortnight , help wipe out all the toxins and lead towards total health .
Eat plain healhy balanced diet as fruit , veges , juice , soups , milk , curd ,Omega 3 , antoxidants -ginger , garlic ,almond, cheese , oats ,dalia, coconut water and take lot of water .
Eating small meals more frequently and chewing carefully will help your digestive system too.
- Regular physical acivity to increase proper blood circulation - walk ,exercise , yoga ,pranayam deep breathing ,further detox your system , help boost your immune system and give resistance from diseases .
Fast foods , refined sugars and fine floor products , tea , coffee , alcohol , smoking , sleeplessness , dehyderation , constipation , worry ,stress all induce toxins in your body and cause you sufferings . So avoiding is always a better option .
Regular check up from your doctor is highly appreciable
Hope this helps solves your query .Take care , All the best .
Don't hesitate to get back if have any further query