I think the Pod has to be around 1700. You need to stop the American independence, because that was what started the proces of decolonisation in the Americas.
It's gonna be difficult, because,at leas IOTL, the patron colonies usually followed was more or less this one: once the colonies had developped to a certain extent, and once the government of the mother-country was percieved more as an obstacle for continuous developpment than as someone who could encourage local developpment (either by providing technology, security or being a trade partner), the push for autonmomy and/or independence startes to gain momentus.
This happened mainly in settlement colonies, but an analogous process occured in the other type of colonies: at first colonisers what resisted, but this resistance often failed due to the technological gap between colonisers and colonised. When locals realised this, resistance stopped for a while (at least openly), and a few of the locals chose to colaborate with colonisers. Once the colonised thought the technological gap had been sufficiently reduced, the push for independence started again (this times based on new "weapons", both technological and ideological).
So, in order to stop the Americas from going independent, you need to reduce their developpment and/or make mother countries being seen as neccesary for the well-being of the colonnies. As carlton_bach has said, if the Anglos see that a push for independence would lead to them being ruled by the French King, they might prefer to stay loyal. The same goes for the river Plate are: if they had to chose between going independent and being conquered by Portugal or staying under the Spanish Crown, they might have chosen the first alternative.
However, this might not be enouigh. You'd need either to strenghten European rule (which would lead in the long run to a very opresive rule), or to make the European colonies independent in everything but in name.