About the only way would be to put the pot next to a sturdy support like a fence post, tie the stem to the post, and very gradually pull it upright, I think.
But, please consider first that plumeria really don't grow "straight" anyway. They are naturally an open, branching shrub. It may be getting ready to put out a branch in the other direction that would balance it out. These plants growing in the wild don't often fall over, they spread out their branches to balance. That little guy just hasn't got his feet under him yet and looks like it put out too much top growth for its stem to support.
Ease back on the fert and water as the weather cools and see what happens next spring.
