As Benajmin explained, both are correct
It all depends on the context of what you try to use those sentences in. take this timeline for instance:
(action A) : being nice
============ AAAAAAA =======>
As you can see by the first sentence or the present continues, you're trying to imply that I can see there is a limited amount of time in which I found him being nice or simply I have recently discovered him being nice.
BUT for the second statement or the present perfect:
(action A) : being nice
(action B) : you seeing him being nice
ABABABABABABABABABAB... =======>
meaning: He has been nice throughout the day and you have been seeing it and feeling it and now you are explaining it to some certain people.
Hope that helps.