The moral issue in this specific case and more generally.
In the specific case: was this MP in Parliament when the Commons voted to hold the Referendum, and, if so, did he vote for or against?
If he voted for[i then he has the obligation not to uphold the result and not try to thwart it. If he voted [i]against] then his current stance is consistent with his original position and I, personally, would respect him for that.
The wider issue: the people who oppose Brexit and want to reverse it believe that Brexit is bad for the country, or for themselves, or both, but they come in separate moral categories. The first and more numerous group despises the Brexit majority and wants to put the stupids back in their place. Naturally they disguise this contempt with rationalisations of varying degrees of sophistication: "the referendum was only advisory", "it was never a simple binary choice anyway", "the facts have changed", "now that they're aware of the complexities..." etc etc etc. This group is doing an "Up yours" to half the electorate, much like Hillary describing Trump voters as fifty per cent "losers" and fifty per cent "deplorables". The less numerous group are so convinced (I believe) of the damage that Brexit will cause that they think they have a moral duty to save the sinners from themselves despite the democratic implications of overturning the referendum result. The analogy I would use for that group is those parents who see their young teenage daughter fall for an obvious undesirable: they "know" that a marriage will end up in misery and betrayal, and do everything in their power to break the relationship up at the known cost of breaking their daughter's heart in the short term. This was the stuff of Victorian novels, wasn't it: young heiress wooed by seductive adventurer; father pays fortune hunter to leave immediately with no goodbyes etc.