I'm no expert here but I'd think you'd want to keep it close. Depending on your pattern density, 25 yards sounds doable. Looking at Winchester's website, their 3" 3/4oz #6 shot load for a .410 is at 1100 fps. A 20 gauge pushing similar loads (but more pellets) seem to be around 1250 fps. I imagine if you have it choked down enough he can take 25yard shots but I wouldn't really want to extend it much farther. Is it possible for him to handle a 20 gauge? My first gun was a .410 and when I started hunting I was small, but I could handle the kick for one or two shots with ease. Good luck, and I hope he gets one.
-- B