I would to insure that the intestinal flora thrives. What I would do instead of the antibiotics, is give him colloidal silver. If the incontinence is caused by an infection, this will take care of it.
If it is merely caused by age, I just finished making another post on incontinence in this forum referring the poster to the following link:
This is a great site with great products for pets.
I would get rid of the antibiotics and switch to colloidal silver and the product at the above link. Even if the incontinence is not caused by an infection, a daily regimen of colloidal silver will insure your pet's optimum health.