Nice job In order for it to stand, I used a coin inside the body, i placed it on piece number 3, worked fine, it's been more than one year since I built it and it's still pretty stable.
Usually, the ones that don't stand on their own have so writen on the notes about the model, as you can see on some models I testbuilt that didn't stand on their own, like Chibi Dratini and Hydreigon, or on the template or on some notes that come inside the download pack. Coins work most of the time, but with some models I needed something smaller, so I used corn, rice, beams... worked fine as well, or you can use a base.