Is Rob Hall's body still on Everest...
Yes. (Unless something very unusual has happened very recently.)
Virtually all the people who died on Everest (or other extremely high mountains) are still up there.
The fact is that it is next to impossible to transport a frozen body down from that height. Aside from the difficulty of transporting that much dead weight in a terrain that kills even experienced climbers on a more or less regular basis, there is the cold, and the lack of oxygen that makes extended stays (let alone work) at altitudes above 7000m impossible. (Read the story of Francys Arsentiev, for an example how even climbers still alive cannot be brought down from the top if unable to move on their own...)
That, together with the permanent cold, means that those who die on Everest are pretty much preserved until wind and weather erode their bodies (George Mallory's body was only found after 75 years...), or one of the few cleanup expeditions gets them down.
As for Rob Hall specifically, there are several pictures of his body, which can be found both in books (Göran Kropps „Ultimate High: My Everest Odyssey” to name just one), and online. I will not provide direct links out of piety (I don't think highly of such pictures), and because not everybody is prepared to see a dead body in a StackExchange answer. There is no record of him having been brought down by a cleanup expedition.
...and did Bear Grylls see his frozen body?
Here, we will have to take his word for it. But as the body is there, its location is known, and Mr. Grylls probably knew the pictures and so would have had no problem identifying Mr. Hall's body, I would say this is highly plausible.