Not convinced it's Japanese Plum. The fruits don't look quite right, nor do the bark/leaves (based on a google search on Japanese Plum). Can you show me some example images? A lot of the immature plums, etc I'm seeing online tend to be narrower at the stem end, and wider at the bottom. This fruit is wider at the top where it is attached to the stem.

It looks like a Praxelis. Do the leaves give off a stink if you rub between the fingers?

My advice? Buy two more..or three more. Plant them about as far apart and in full sun as you have. Because the wait for one tree to give you many fruits is going to take at least 6-10 years at best in Fremont. If you plant four or five trees? You will get more fruit without waiting for one tree to reach 15 or 20 feet. I planted mine in the ground in 2012 and its about 12' tall. Full sun,reflected heat of a house or wall. NO SHADE!..if you can help it. Lots of summer water,feed them when they flush. Remember in summer...water,water,water. Soak deep.. THEN let it go a little dry in between When they show they like what care you give them.by flushing well,keep up that good routine of water and feed.. Here's my Hayward Manila.

Could you circle these tendrils because I still don’t see them however much I enlarge the picture? And please could you give measurements for the leaves? There’s nothing to give a sense of scale. Meanwhile compare with Abrus precatorius.