I don't see a single POD that could bring all of British North America into the United States at one time. We'd probably need an early POD that generate butterfly after butterfly.
A good start would be in the very beginning of the struggle for independence. If Nova Scotia sends a representative to the Continental Congress, it greatly increases the chance that it will become part of the US (even if Halifax itself remains occupied for the entire war).
Then, of course, you could have a different outcome of the original invasion of Canada. Even if Quebec does not fall, you could have the US keep enough troops that Montreal is held.
Washington had several opportunities to mount a second invasion of Canada which would have been much better supported than the original attack. IOTL, he decided against it because the risks were too great - he wanted to be sure all the 13 Colonies would be kept rather than risk gaining new territory. In a scenario where Nova Scotia and/or Montreal is considered American territory, Washington might consider a second invasion which will either secure those territories or even get new ones.
You now have a good situation where some of Canada is now held by the US after independence. The next step would be gaining additional land in a War of 1812 scenario.
Britain will still have King Rupert's Land and a claim to the Oregon territory, but having lost the crown jewels of Canada, these are not as valuable alone.
So the next step might be a stronger US push for British Colombia during the Oregon crisis, and Britain decides appeasement is fine.
At that point, Britain may not see much use in keeping any claims to King Rupert's Land and begin negotiations to sell the area shortly after the Civil War.
So in the end, Britain keeps Newfoundland and Prince Edward's Island, but everything else is part of the US. This requires a lot of good luck, but it is not impossible. For a plausible POD for Nova Scotia to be more in line with the other colonies requires some work.