Dad's money is being taken by son who is not POA, for his own purposes. Any advice?

Agree with eyeirish. Take control of your dad's funds. Is your POA standing or springing? Standing is in effect now, springing he has to be incapacitated in some way which does not sound like it is the case if he is just now beginning to become forgetful. It may be time to make sure that you have what you need in order to help your dad. You could always tell your brother to stop and let him know you will contact APS. You could even mail notification to him certified, return receipt telling him what will happen if he continues taking money from Dad. And the problem with APS, if Dad is competent he can do what he wishes with his money. Would dad tell APS that brother is taking advantage of him and that he is fearful of brother if he did not give it to him?

Call Adult Protective Services (APS). It may go by a different name where you are but Google it in your area and call them. As POA you'll be expected to provide proof that your brother is financially exploiting your dad.

Take dad's checkbook away, cut off the flow of money to the brother. Give dad enough cash on hand to make him feel like he has some control but not enough to feel the pinch if he gives it to your brother. Take his debit card away, his credit cards, everything that will allow your brother a path to the money.