Coaster brake brakes itself
Are you sure that the friction comes from the hub, not from tyre rubbing against frame? Axle mounting point looks as it is not tightened properly.

Second thing to check is to assure that hub is not screwed too tight.

I would personally unscrew arm-side of the hub and check the clutch and its "spring" condition. It's easy, no special tools required. Check the very first minute of this video.