It appears, from your description, that your father is aspirating thin liquids, which is why his physicians are recommending a thickener. Having aspiration can lead to aspiration pneumonia
, which can be life threatening. The only other solution to this problem is to stop allowing your father to eat and drink. Nutrition would be accomplished by placing a PEG tube in his stomach which will allow him to receive nutrition bypassing the mouth.
Be well, Dr. Kimberly