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!
Why can't you get a small .22 single shot bolt action such as the Chipmunk or Crickett. They wouldn't be noticeably more than a good quality air rifle, accurate, and lightweight as well as being safe. Also the higher power air rifles are HEAVY and HARD TO COCK on the break barrel versions. Get the best you can afford in air rifles. Word of advise-use heavy, good quality pellets instead of the lighter Gamo pellets. Basically when firing the Gamo Raptor pellets, it's almost like dry firing a bow. heavy pellets will penetrate deeper and generally be more accurate.
And this grandson is how old? Cricket or Chipmunk or whatever it's called these days is designed for Grandpa . . . The gun is too small for most kids over the age of 8 -- which is the age where I'd be thinking about introducing a kid to shooting a 22 cal.
Marlin makes the 915Y -- scaled down single shot bolt action. I picked up one used for $99. But wouldn't let any kid have any sort of rifle . . . BB gun, etc. until he's about 14. Unless with adult supervision during the possession and use.