hello im a dr and this is my advise
39kg for an adult male is too low, is there any reason he is not feeding well? food factor? toothache? There must be a reason why he is not eating.
I would recommend you to bring him to see a dietician, as there are various products out there that can help with weight gain in those with poor appetite but do find out the reason why ur dad is not feeding well.
U can also bring him to a geriatrician, there are certain medications that are available for pts with dementia
Another thing that i would like to point out is if u noticed anything wrong with ur father, for instance he is looking more tired than usual, drowsy, sudden change in behavior, pls bring him to hosp immediately as old ppl tend to deteriorate very fast