I doubt he can Transfigure antimatter. If he can, the containment will be very hard to get right, and he would absolutely have to get it right. How do you even stop it blowing up your wand, if you have to contact the material you're Transfiguring?

Transfigure the containment device first. Then find a way to transfigure the antimatter inside it. To solve the wand contact problem, transfigure the empty container into a full container. Then his wand is in contact with the container, but the container doesn't actually change.

Granted, it's extremely dangerous, especially when practicing.