What are chances of being turned away at the border?

My Canadian boyfriend wants to visit me this summer in the US. He would like to stay 2-3 months. We are not planning on getting married at this point, and neither of us has made any decisions about moving. I am a fulltime grad student and part-time freelance writer. He is a retired attorney, although he's young--45 yrs--and is afraid that the border agents won't "believe" he doesn't work or intend to work in the US. (He's basically an independently wealthy loafer and has the opportunity to hang out with me for awhile during my school break.) His fear is that he will give up his residence in Canada (he rents monthly) and sublet a place here, and risk getting turned away at the border and have his plans ruined, plus no place to go back to and lose money on his US sublet. My opinion is that he won't get stopped, and even if he does, if he simply tells the truth and has documentation to back up that he doesn't need to work (like a huge bank balance) they'll let him go on through. He says the border officials give him a second look all the time, but he travels here every few weeks for long weekends--it makes sense to me that they give him a second look, but I still think it wouldn't be a problem to visit here for a couple of months. Any advice on documentation he ought to provide or his risk in being denied entry. (ps: he usually flies b/t here and there, but in the summer he will be driving so that he has a vehicle while he's here.)
Thanks for reading!