Air rifle for Grandson

A used RWS 34, HW 30 or HW 50 would work. A used HW may get you in your price range. I like the RWS 34 as most bang for the buck,. Good accuracy, good quality and power enough for small game hunting or pest control.

https://www.airgunsofarizona.com/spring-piston/rws-model-34/ -$255

Flying Dragons use to have a RWS 34 Chinese clone from Xisco that was cheaper and good quality. Not sure if they still do as previous poster said website is down for reconstruction. Suppose you could call him. I would pay to get it supertuned by Flying Dragons which ups the price. Highest UTG scope you can afford as far as scoping any airgun. I like the UTG 3-12 Bugbuster.

As far as a pumper, I would get or borrow a cheap Crossman 760 or better yet the Daisy 880 from Walmart and see if he likes it before going for a quality pumper. The pumping does get old but is fun for me as it reminds me of my childhood as I have the same exact guns I had or wanted as a child. If I was just starting out in airguns, I would have gone solely with springers.
The XS25, now only in a synthetic stock last I saw. Still a very good rifle with Mike's full tune and 2nd screw trigger mod, plus his warranty. I had one in .22 with a wood stock. very good shooter, not pellet picky and pellet made more noise hitting my back stop than the rifle made shooting it. Would leave plenty for a a decent scope and pellets.
Can't argue with a pumper either owning a Silver Streak but I'd look for a Benji 392 or 397 so he could buy pellets local instead of online. hAlways a good workout but he might get tired of the pumping part at some point and wish you'd got him a springer instead...lol.
Can't argue with a D34 owning 2 myself or the HW30/Beeman R7 I also own 2 of. Would just be finding a good used one and likely not cheap from what I tend to see people asking lately.
Lots of good ideas being posted, don't envy you trying to decide.