I.G.
If they are like my DD, never - and she is a perfectly "normal" 5 year old... just stubborn as heck.
If your son is otherwise fine and making normal progress in his development (and it sounds like he is) you don't need to be concerned.
One symptom doesn't make an autism diagnosis, as a matter of fact I would say that probably most, if not all perfectly healthy and "normal" kids show at least one "red flag" for autism, adhd (or insert any other developmental syndrome in here) at one point or another in their life.
Good luck!