I'm not a physician, of course, but it's important for your husband to be evaluated by a doctor to be sure his memory problems aren't caused by a more serious underlying condition. Even if they aren't, your doctor may suggest some ways to slow their progress.
How can you get him to agree to see the doctor? It may not be easy; perhaps he secretly senses he has a problem and is fearful of having it confirmed. He may also fear losing his independence (such as being required to stop driving). However, elsewhere in your letter you mention that your children also have noticed he's having these problems. I suggest you enlist their help -- not only to encourage him to see the doctor, but also to help you plan for the future.
Most of us fear losing our independence as we grow older -- but that time will come for most of us if we live long enough. Now is the time to face this fact and prepare for it, instead of waiting until others have to make decisions for us. The Bible says, "Remember your Creator in the days of your youth, before the days of trouble come" (Ecclesiastes 12:1).
May you both also discover new strength from God during these days. Old age isn't easy -- but God is faithful, and He hasn't abandoned you. Turn to Christ every day for the strength you need, for He has promised, "surely I am with you always, to the very end of the age" (Matthew 28:20).