My father's first symptoms were the start of a "floppy" left foot and then a weak feeling in his arms that he (and his MD) thought was the start of arthritis.
My father was recently diagnosed and we were shocked. I pray this isn't the case for your father but my father also was diagnosed with arthritis issues, then they did 100 tests because they thought it was a spinal thing and we just wanted the real story so he could start to be better. Well, it wasn't good because he does have ALS. I would suggest giving the forum more symptoms to find out more for you.
My father's first symptom was his left foot 'dropping' when he walked. That was in Feb of '05. By July '05 he has the twitching with muscle atrophy and weakness. Please give more symptoms....everyone is different. Some people start 'bottom (feet) to top' and others start in the bulbar region 'top to bottom'. His Dr. first thought of the statin drugs.
I wish you much luck. It can be a long road.