The Microsoft Optical wheel-mouse isn't cheap, but it is very tough - mainly because it doesn't have a rollerball underneath. What it does have is a strong plastic case and base, and a pretty little red tail-light. That alone should have considerable grandson appeal, and might encourage him to be more gentle.
I use mine constantly - one for each PC and one for the laptop - and I haven't managed to wear one out/break one yet.
Thankyou all for your advice. I have ordered Dicmay's chic optical mouse and an even cheaper roller ball job from ebuyer [in case we have probs with the optical one]. Normally I keep three spare mice but shall stick at two until I see how they do. The multimusicide household of my descendants had to turn to a logitech job that looks like something off the pudding trolley with a purple ball/cherry on top after they had raced through four mice in quick succession. This would be a solution for me too if it wasn't so fiddley. Finally I'm off to find out what a PS2 mouse using a serial to ps2 adapter is/does.