If you have a close personal relationship with him, and others around you are seeing this decline, can you confront him with the issue? Explain, as you just did, that you are wondering if there is a medical problem, or if there is something going on that you should know about. Give him some examples of specific incidents that have concerned you.
Speaking for myself, I can tell you that I was recently on a medication which was really affecting my memory, my emotional state, and my performance as a whole. I had made a number of bizarre mistakes in my work, but when they were pointed out to me I blamed my boss for being so critical. Couldn't I be allowed to make one or two mistakes? It was not until the Christmas break that I realized how much it was affecting me. I went off of it, but it took a while to clear my system and my boss took me out to lunch in January to find out what was going on. I knew I should have approached him about it sooner, but I just couldn't figure out how to go about it.
On a similar note, we are working on two cases right now where a person has started to lose their short-term memory. We are trying to help them to get their estates and businesses in order in anticipation of further degeneration. One of them just told his secretary this week what has been going on, and I imagine it is a great relief to her to know what is going on. She now knows what the problem is and can help him to stay on track until everything can be sorted out.