06 is more than enough...knwo your distances...have a range finder good out to 400 yards on deer sized game...and print up a little sticker that you can mount on your scope to tell you drop for distances...will help your son remember and remind you in heat of moments...is a simple little trick, but works VERY well for younger/inexperienced distance shooters...

as to how far...Would HIGHLY recommend practice practice practice for distances out to 400 yards...and then discipline yrouself to get under 200...but 75 would be a very short shot for open country...