There's no *direct* evidence. Orthanc, Isengard, and the Stone were of course the property of Gondor and therefore of the King. In that very chapter you cite, Aragorn makes a big deal of specifically ceding Orthanc and Isengard to the Ents:

>[Gandalf:] 'But the Tower of Orthanc now goes back to the King, to whom it belongs. Though maybe he will not need it.'

>'That will be seen later,' said Aragorn. 'But I will give to Ents all this valley to do with as they will, so long as they keep a watch upon Orthanc and see that none enter it without my leave.'

He says nothing specific about giving the palantír to anyone—in particular, he certainly seems not to have given it to the Ents (who probably wouldn't have wanted it in any case); and it seems unlikely that he would have just left it in a locked building. The context of the quote you give in your question, however, may give a clue:

>'Only one now remains that you could use,' answered Aragorn, 'for you would not wish to see what the Stone of Minas Tirith would show you. But the Palantír of Orthanc the King will keep, to see what is passing in his realm, and what his servants are doing. **For do not forget, Peregrin Took, that you are a knight of Gondor, and I do not release you from your service.** You are going now on leave, but I may recall you. And remember, dear friends of the Shire, that my realm lies also in the North, and I shall come there one day.'

(*Return of the King*, Book VI, Chapter 6, "Many Partings"; emphasis added)

The fact that Aragorn first speaks of a king watching what his servants are doing, and immediately follows that by a reminder to Pippin that he is a servant of (that is, one in service to) Aragorn's kingdom, and that Aragorn may have cause to know what Pippin is doing and summon him, strongly suggests to me that Aragorn is speaking of himself here.