It is normal to become less tolerant to alcohol as you age. In addition, if your dad has been a heavy drinker all his life, he may have a bit of liver damage. In addition the time he spent recovering was probably 'dry' and may have had the effect of making him less able to take his drink when he's started again.
I woudl definitiely ask around for support and help - via your GP, Age Concern, and Al-Anon (for families affected by alcohol issues). There may be an issue, depending on your dad's doctor, of his autonomy - ie his doc may refuse to assist you, if you are making the request and not your dad.