Before investing in one of those air guns pick up those aguila super colibri for his 22. I have been playing around with these lately and I am most impressed. Its basically a 22 lr with only the primer, no gunpowder. Just as quiet as the airguns I have and quieter thanthe really fast ones yet it delivers a 22 caliber punch without pumping or cocking. Use them in my son's cricket rifle and a henry 22, no semi autos as they won't cycle action.
If you insist on an air gun then the sheridan 20 cal pump up has always been my favorite. Their 20 cal slug always seemed deadlier than anything else out thereand was always real accurate. take care.