Applaud the idea, and I think watching him at the range should give you an idea of if he is ready, especially since he's your brother
.
I would suggest upping it to a .243. There shouldn't be that much difference, and he would get more use out of it as he ages. I just think that a .223 is a marginal deer rifle (although a properly placed shot will do the job), while a .243 is a great deer cartridge. Once he gets bigger he will do what most young men do and want a bigger rifle
