1000fps. or more...
Look for break barrels instead of CO2 powered... I like the GAMO brand, I had one of those and are indestructible, reliable, long lasting, exelent craftmanship... but you could look for Crossman brand too, I had a Crossman pistol and I loved but was a CO2 and sometimes run out of gas, that's why I do prefer the break barrel... and .177(4.5mm) it's a good caliber but I still prefer the .22(5,5mm).
I hope your grandson enjoy it!