Floating Ray: Is that okay for him?

No, this is not okay for him.  This is usually due to poor water quality or lack of oxygen.  This stingray keeper experienced this behavior after feeding oily seafoods and upon water changes the stingray returned to normal behavior.  Notice the frothing bubbles, not a good sign.

"Potamotrygonidae developed at least two interesting adaptations to freshwater environments: the first is the ability to floating on the surface when the bottom water was poor in oxygen and the second is maternal care. These adaptations are unique among Elasmobranch species (Achenbach & Achenbach, 1976; Araújo, 1998)."

Also, low Oxygen will cause this behavior.  Surface Agitation causes gas exchange, so find a way to create a ripple at the surface. Higher temperatures lower dissolved Oxygen, and Rays prefer high temps.