i don't know much about elk hunting, but the yardage is similar to deer- i don't think that that would be a good range to shoot in- i think 500-something yds would be your limit, maybe somewhere around 600 or pushing 700, but at 1000yds. the bullet will have dropped so much and it wouldn't be 'traveling' as fast as it would be if the yardage was closer
also, a 308 wont be much for your son. i'm 5'1 and 115 lbs and i can shoot a 308 with no problem- i'm guessing your son is probably about my height or taller (since a lot lot of people are taller than me lol), so he'd have no problem
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added:
even practicing at that range, you'll have to shoot a lot because of the way the wind blows- different each time, and how much the bullet drops- which is different each time also because of the weather- humidity, temp, etc, and who is shooting it makes a difference
also, you did NOT tell us that you'd be practicing at that, you didn't say one way or the other, next time please tell us if your gonna be hunting or practicing at those ranges instead of calling us idiots [wasn't wanting to sound rude, just pointing something out]