Dad can drink for 25 days, that is, dad drinks 1/25 of water every day.

Father and son can drink for 20 days, which is 1/20 of the water that father and son drink every day.

So,

The amount of water that a son can drink every day = the amount of water that a father and son can drink every day-the amount of water that a father can drink every day.

That is, the amount of water that a son can drink every day =1/20-1/25 = 5/100-4/100 =1/00.

Therefore, the son can drink1100 water every day, that is to say, the son can drink100 days in this bucket of water.