My 75 Year Old Father Is A Very Healthy Person.

My 75 year old father is a very healthy person. he rides a bicycle nearly every day. Golfs, works out at the gym. Still goes to work 4 days most weeks. Travels extensively. In the past year I have noticed that he speech has been slurred and slow. He has been tested for a stroke and it was negative. He has seen a neurologist and his MRI was clear. My great grandfather (his father) has Alzheimer's. Is this a since of Alzheimer's?