If it were for your self I would say 257 but for your son the 6.5x55 would be a nice alternative. If the stock can be refitted longer as he grows or a second stock in a few years then it could become a life time rifle. The 6.5 is a bit more versatile than the 257 especially if game larger than deer are factored in.

Ammo can always be ordered but off the shelf availability is a nice option if he doesn't reload later. If the choice were based on ammo options then the 243 or 6.5 Creed would be the top choices as other have posted. My sons I did a 22lr, 223, 243, 270 progression. One son decided he didn't care to hunt the other is an avid hunter shooting a 270 Nula/Colt light Rifle. Both still like to shoot.

The 6.5x55 Forbes rifle has become my go to rifle even though I just finished two others recently.

"When you disarm the people, you commence to offend them and show that you distrust them either through cowardice or lack of confidence, and both of these opinions generate hatred." Niccolo Machiavelli