It has no taper, could be fatter, suggestions?

The Grewia will probably never have any more taper than it does now. I believe
you are in Zone 9, and I don't think Grewia is frost hardy. You might get the
trunk to thicken a little by growing it in the ground in a frost-free location,
or putting it in as big a training pot as you can carry. With Grewia, what you
see is pretty much what you get.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"When you come to a fork in the road, take it." Yogi Berra