You need to be able to distinguish between excessive thirst leading to excessive amounts of water being drunk which leads to not being able to "hold it all in" and incontinence - leaking irrespective of bladder fullness.
Incontinence medicine usually works on females by tightening the sphincter muscles and hence stopping leaking. It doesn't work so well on males.
If your old boy is drinking so much that he con't contain himself especially when he is asleep then it may be something else.
The most common is probably diabetes but there are a whole bunch of other things that could cause this.
Also, if he is drinking excessive amounts of water, then his blood and urine tests may come back in the normal range which actually indicates a problem.