Since he's 5 hours away, and your hunt is a year away, if you plan on a Christmas visit THIS year, IMHO it's a good chance for a heart-to-heart/face-to-face discussion with him, as to what firearm to bring.
If not, I would advise a nice, long phone chat about your/his plans for then.
Asking for HIS advice would go a long way towards a smooth hunt, to say nothing of familial relations - AND, you'll most likely get the straight poop on what works best for what/where their particular hunt(s) are planned.
Be up front, about your rifle inexperience - then get an effective, but relatively soft recoiling rifle and PRACTICE with it for at least the next 9 months before your hunt.
Since you mentioned a shooting hause/hut, one can presume they're set up near some open areas, suggesting that long shots (say 200+ yards) may be the norm (ask your FIL).
A lightly scoped (4x or 6x) .243Win, .260 Rem, or 7mm-08 bolt action should be more than adequate.
AR's while very acceptable by younger shooters & hunters, are not so readily accepted by older hunters, and a newcomer showing up with a new one would in all likelihood not be asked back, relative or not.
It sounds like you friendly FFL is a small urban shop that caters to the Black Rifle trade, ergo his recommendation - not necessarily the best for your situation.
I would further urge you to take a ride, and cruise some outlying gunshops, for a different take on what's available - and don't turn your nose up at their used gun rack(s), since many lightly used/fired specimens, of whatever, may be found there.