Why do you think Loras squired for Renly? Was Jon Arryn involved or was it a move made by Renly himself?

No, I don’t think Renly would have made any such moves, given that he would have been quite a young man when Loras began squiring for him, probably about 15 or so.

Rather, I think it was probably a move by Jon Arryn – it gives the Tyrells a carefully-measured bit of royal favor (a third son for a third son), while giving the Baratheons a hostage for their good behavior.