That CZ your son is shooting is a great sporter class rifle. Buying a T200 would be basically buying the same gun, I can't see where there is any benefit unless it's having one of his own. The Daisy 599 is a smoothbore BB shooter for that specific type of competition, would never hold a candle to the CZ/AA S200. I'm not familiar with the Hammerli. Another rifle shot in the same class as the CZ/AA is the Crosman Challenger, they just came out with an updated model in the last year. But I would still take the CZ/AA, those guns are gem.
Honestly, I would talk to the 4H shooting coach about what they recommend for the next gun. They might steer you toward a true 10m competition rifle. Start stacking your dollars, because an older but good used10m PCP will probably bite you for $1200 or so. Here is a link to an FWB800 new on AoA:
Time to start mowing lawns!! (Do kids still do that?????)