I went through this last fall with my 10 year old.   He ended up with a .54 cal round ball with 60 grains 777.  While practicing/working up the load, his first shot was only 40 grains - I knew it'd be important for his 1st impression to be 'that didn't kick too bad - I can handle this'.  After that, getting him up to to 60 grains wasn't hard at all.  He took 2 nice little does with this load - 1 at 90 yards & the second at about 50 yards - both pass thrus.  A rest is a must.
I couldn't disagree more with making a beginner use iron sights.  You want him to succeed, right?  Why withhold the best tools?  Take him out shooting with an open sight .22 for practice, but give the kid a break when it comes to live targets!!  How are you going to feel if he wounds one with open sights, on a shot you know he coulda made with a scope?