A notification under a treaty is a Royal Prerogative act.You can listen to his comments here, starting 6 min 50 seconds in.
Triggering Article 50, which is a formal communication to the European Council stating the UK's intention to leave the EU, is a notification under a treaty. Therefore it falls (rightly or wrongly) under the Royal Prerogative, which means Parliament does not need to ratify the action or be consulted about it. The government can act unilaterally to trigger Article 50.
Many MPs are declaring that Parliament must be sovereign (something they have seemingly not bothered about while allowing political and legal primacy to reside with the EU and the ECJ respectively). They are right. Parliament should be sovereign. Royal Prerogative is not democratic. So how can it be right, therefore, for someone like me to say that the prerogative should be used to trigger the Article 50 notification?
MPs are saying that Article 50 must not be triggered without their approval and that the prerogative should not (in their words 'does not') apply. But this is where alarm bells ring, because it seems this sudden desire for control is only emerging now the Brexit process they fought to prevent is set to begin.
MPs would had have a valid point about Royal Prerogative being anti democratic, generally and in relation to Brexit, if the House of Commons had not rejected David Davis' 1999 attempt with the Parliamentary Control of the Executive Bill to curtail the use of the prerogative by transferring ministerial exercise of it to MPs for a number of scenarios - including, ironically, the signing of treaties.
Voters can be forgiven for thinking that MPs want to challenge and rein in the prerogative only now because they want to undermine Brexit, or at least decide its terms. This is an attempt to change the constitutional process on the hoof for the sake of expediency, because they don't want an outcome that British voters decided by a majority of 1,269,501 to pursue.
It is perfectly reasonable to ask MPs how prerogative can have been perfectly acceptable to the Commons previously when they rejected Davis' efforts to curtail it, but suddenly be wrong now just because it is the constitutional mechanism that would be used to trigger the Article 50 notification they oppose.
The challenge to prerogative being used to trigger Article 50 is a naked example of political double standards. If MPs want Parliament to be sovereign they should change the constitutional process formally, in the proper manner, not try to kick it into the long grass arbitrarily because it suits their purpose in regard of this matter. They must not be allowed to subvert or frustrate a decision taken democratically by voters, because the issue was too important for Parliament to decide without formal mandate. This is why Davis is opposing something he previously pushed for.