The douchebag is a legal stranger to the matter and notice has not been provided of the move - the mother has complete deniability that this is going to actually occur.The Step Dad sat me down yesterday and told me he will be accepting a job in London for the next two years planning on taking my son with them.
Any motion brought by the father now will be dismissed. Something has to actually happen.
If the mother intends to move the child, she must file a motion to set aside the geographical restriction, that is the time to counter the effort. Right now, the OP only has gossip, rumor and innuendo. He does not have anything from the mother. It is entirely possible that the mother has told her husband that she cannot go because of the child, and the husband took matters into his own hands.
If the mother were to remove the child without legal authority, it has to actually happen before relief can be sought. The husband, not a parent, would be engaging in kidnapping, and would be in immediate trouble up to his eyeballs. And think about this - If the job in London is with any reputable company, it will not tolerate it's employee's engaging in child trafficking. This fantasy that the child could be removed to London and leave the OP wringing his hands in frustration is a movie script, not reality.
The OP is in a comfortable place, and has everything going for him and his relationship with his child. His efforts now should be to consider how the child's visits to the mother are going to work, and might even consider traveling with the child at the mother's expense - Dad and child can be tourists on a lifetime adventure.