I am not sure that I understand your question. What do you mean when you say that he gain weight overnight and lose weight overnight? That isn’t possible in a real world so maybe you haven’t wrote it correctly. Would you please elaborate?
And if he is drinking it is quite possible when you see him that he is swollen from alcohol effects so maybe it seems to you that he has gained some weight. And if he is drinking when he is depressed about his relationship status this is bigger problem than gaining or losing weight.
If you want to help him talk to him about his problems because weight is something that can be corrected and depression and alcoholism are serious conditions.
It is very nice that you want to help him but first find out what is the root of the problem and then try to do something about it.