When my son was 5-6 I bought him an Anschutz Woodchucker. What an awesome little bolt rifle... 5 shot clip, great open sights but also scope ready, adjustable trigger and a beautiful high grade wood stock. Would shoot as good or better than even my custom rimfires. Unfortunately they were only made in the mid 80's (when my son was 5-6), but if you could find one you'd be giving your son a heirloom he'll cherish all his life like my son does his, and pass on to his son.