If one parent has a very mild form of autism, would his child most surely have autism?

Not sure. Autism is a disorder that is very difficult to attribute to one specific factor.
No. There is some evidence of genetic loading but clearly not one to one.
Seek evaluation. This is an evolving area of medicine and you question is a good one. First, you say mild autism, but that sort of label is a confusing one. How and why this diagnosis was made should be looked at. A meeting with a medical geneticist can review this and see if the label is not covering up a potential inheritable condition. If so, risk could be as high as 50%.