My five year old shoots my PCR1 off a bipod, she can shoot the Contour unaided. So maybe an HW100 with bipod until he muscles up a bit?
I agree that if he's got his heart set on an HW100 then whatever else he gets won't do it for him. Probably best to go to one of the gunshops that has a "try before you buy" range, or better still go to your local HFT/FT club - there are bound to be HW100s, S400s, S200s and others for him to try.