Protein shake recommendations?

my 78 year old father has lost a lot of muscle recently. he also has lost a lot of teeth (also related to poorly controlled Type 2 D) and is not getting this taken care of due to some psychological issues (not financial as he is very well-off). i thought that drinking a good quality, high protein, sugar-free shake once a day in addition to more whole food, easy to chew protein sources (like sardines) would help him. any recommendations that you have personally tried?