Personally, I think he was a nutjob.  Notice in this article how he changes his numbers several times regarding how long the earth will last or mankind will last, yada yada yada. 

Did he have polio from a vaccine rather than ALS?  It would seem so.  But the headline of this article is misleading because the article seems more concerned about Hawking's opinions about God and life than it does about the polio aspect.

Nevertheless, it's good reading.

https://www.lewrockwell.com/2018/04/bill-sardi/stephen-hawking-the-oracle-at-cambridge-who-may-have-been-a-polio-victim-from-a-vaccine/