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%.