Defnately go with a bolt action Rifle.

My first centerfire rifle 30 years ago was a 243 which I used for a few years,even saw some large deer dropped convincingly with it but i would consider it a little light for deer,then came the 25-06,a great cartridge but loud & has a large muzzle flash especially in a shorter barrel,next came the 270,what a great cartridge I thought,until a friend bought the same rifle in 7mm-08,at first I wouldn' t believe A cartridge with the same case as my old 243 shooting heavier bullets would be any good but at the range I was impressed with its accuracy & lack of recoil & how much more pleasant to shoot off a bench it was than my 270.

On that first hunting trip with the 7mm-08 I was so impressed that I traded my 270 in on one,it would cleanly drop large pigs out to 300 yards,easially as good as the 270.

I also played around with a 30-06 & 7mm rem mag for a while but would consider them way over the top for a 12 year old.

I' ve just bought my 3rd rifle in 7mm-08 a tikka continental,i' m mainly going to use it as a silhouette rifle(knock down metal targets out to 500 yards)

I do not believe that there is a deer on this plannet that cannot be taken cleanly out to 300 yards with a 7mm-08 & the right bullet selection.

I have not used a 260 so have no experience with it but from what i' ve read I beleive it would also be an excellent deer cartridge out to 300 yards or so

For your grandson I' de look for a bolt action rifle weighing around 7 to 8 pounds with a barrel no shorter than 22 inches In 260 or 7mm-08.

If you consider a 25-06 get a barrel at least 24 inches long.
I think a 270 might be a bit much for a 12 year old.

Sorry to be so long winded but hope this will help out with your father.

What cartridges were you thinking of?

Good luck

Tumbo