1) Ask your uncle...it's his gun, so nobody else would know better.
2) CCI mini-mags are about the most dependable in rimfire pistols in my experience, so that's a good place to start.
3) Experiment. Maybe it'll function great with bulk ammo, maybe not. If the gun is in good working order, the massive power of a 22lr wouldn't have me concerned about a kaboom. 22lr is cheap enough that you can buy a few different flavors.
Stinger/Expeditor type rounds are probably the last place I would start, personally.