One version of the correct answer herein, do not look in here before you've answered, or you'll always know you aren't worth the title of Rules Lawyer:
[ Spoiler ]
My printing of SR3 explicitly states that you have to exceed the 10 boxes of Physical Damage on the condition monitor, ie the Physical condition monitor must overflow before additional damage is dealt every (Body) CTs. Since this is not Errata'd anywhere, the canon answer is obviously that he's stable at 10. The passage does not once say "once you hit 10 boxes" or "after taking a Deadly wound", it's all about taking more than 10 boxes.
The way the Deadly Wounds and First Aid section is written you get an idea that maybe that's not how it's supposed to be, that maybe they were trying to say that you start to get additional damage already once you get 10 boxes. But that's not what the rules say, so the undisputably Canon answer is "No additional boxes -- he is stable at 10."
But then you probably knew that already...
NB: I do not in any way or form endorse strict adherence to SR canon.