It can take up to 6 years to mature before it bares fruit. Also, getting a second tree will greatly increase the yield. They do much better when they cross pollinate.

I have to say that if there hasn't been any blossom then it doesn't matter how many other trees you introduce then you won't get any fruit. You could give it a really heavy prune in the summer when there is no problem with silverleaf and a good dose of potassium. It could be that it is so happy with its growth that it doesn't need to fruit. If it doesn't respond next year, I would call it a day with this tree.

Plums bloom on wood that is 2-3 years old, so a hard prune is unlikely to help it bloom. I would first make sure that it is getting as much sun as it can get--being overshadowed by a larger tree would keep it from blooming. As Jimmy says, more potash and phosphate might help it to bloom, too. Bathgate also has a point, in that the tree is barely old enough to bloom, so you may only need to wait a year or two for bloom. Reducing nitrogen fertilizer may speed things up a little, such as skipping the area around the tree next time you feed your lawn, etc.