Slowing his speech and moving slower this year. could grandpa have parkinson's or just old?

Slowing down: It could be both. If you are concerned then take him to the doctor for an evaluation.

Possible-see a Neuro: Physical symptoms can include tremor of limbs at rest>in posture or in action; slow movements & stiff (rigid) muscles causing quiet facial emotions, softer voice, bent forward head & neck posture, decreased arm swing, slow-shuffled walk, small/scratchy penmanship, etc. Non-physical symptoms can include cognitive decline, depression, anxiety, sleepiness, constipation, ed, low bp, acting-out dreams.