Free advice, stay away from counterexamples and hypothetical situations. They detract from the important points that need to be discussed on the field (at the bar, here, when you have time, they're fine and can be helpful, but not in the moment. Umpires need to remember this more than coaches do.)
The first thing is always to make sure that the judgment piece is removed. In this case, ask him if the runner was obstructed, and then if he would have been safe at home if it hadn't been for the obstruction.
If no, you have nothing else to go on. If yes, you now have a rules issue (and make sure you know the relevant parts of the rule and their outcomes.) In this case, we have obstruction with the proper penalty being whatever would nullify the obstruction. I would state that, not ask about it--there's a bit of psychology involved in that if you ask a person if you have something correct, it is viewed as more adversarial than simply stating it.
If he does not agree, now you protest.