Upvote:3
You may make adjustments to your itinerary after you got the visa.
You may not misrepresent your itinerary to get the visa. You should take care that you do not create the appearance that you misrepresented the itinerary, even if the changes came truly after the visa was granted.
- Entering the Schengen zone in Germany is perfectly OK. He would have documents of immediate internal travel to Amsterdam if asked.
- Doubling the intended stay looks bad.
- Going to Switzerland and Italy may look bad. Making day or weekend trips from Amsterdam to northern France or northern Germany would be a nice example of minor adjustments to the itinerary, but going as far as Rome when the original itinerary said nine days looks as if Italy was the plan all along.
- There are no systematic internal border controls. If you go to Italy by car, you may never be checked. Random spot checks will be more likely on the way back north, but those wouldn't stamp his passport.
If I were you, I'd probably do no more than one weekend out of the Netherlands, and add only a couple of days. That's the conservative answer. It would be no overstay if he stays the full duration and travels all over Schengen, but it might raise questions during the next application.