He's a fantail
You can easily tell the difference between fantails and ryukins on the arch of the back, ryukins have a very prominent hump as opposed to fantails. some people are confused when it comes to fantails with deeper bodies or larger bellies, but the difference lies on the backs.
Like on these pics, here's a ryukin:
and here's a fantail:
It's very common for shops to have the fish mislabeled, no surprise since they think goldfish should be kept in bowls...
Please post pictures anyway! I bet it's really beautiful