I know we're talking about quiet, but since the thread title is "ideal sniper sidearm" I feel obliged to bring up the other side of the argument of "loud and proud". Yes the TM mk23 is, in my opinion, the best quiet sidearm, but the only time we use our sidearm when sniping is either on the move or if somebody gets too close and even then you're likely not taking out 2+ people if they're close to you with a mk23.
So I introduce to you the other side of the debate and say that a compact SMG like an mp7 or mp5k or p90 would also serve as a good sidearm for a sniper. Nowhere near as silent, but if you're moving and don't want to give away your position, trust in your stealth and go to ground. If you're moving and you want to engage, you'll have a much better chance of winning that engagement against 2+ people (though the mk23 is better against 1 person by themselves for sure). Similarly, if they're within your MED and they don't know where you are, you can go to ground and wait for them to pass, or if they do know where you are again you're far more likely to win that engagement with an SMG than a mk23.
Basically, a mk23 is a great pistol and I have one and would highly recommend one as a sniper sidearm if you want to go quiet. It's by far my best stealth sidearm and I used it essentially as a primary weapon with a Cyma spring tri-shot as my "secondary" at a private game at the UCAP tunnels; was hilarious people having no idea where they were getting shot from and it went well with my costume as a cyberpunk/noir corporate operative silently taking people out. Even managed to knife kill the juggernaut

(knife kills were the only way to temporarily remove the juggernaut and his scary full auto LMG from game). However, it's awful at any kind of quick shooting because the trigger pull is long and squishy thanks to it being a NBB. I also have an mp7 that I carry in a thigh holster, an mp5k I carry on a sling, and more GBBs than I can shake a stick at because I love pistols and I would say they ALL have their uses as sniper sidearms (even if GBB pistols have their main selling point as "they're fun and cool to shoot")
Anyway, feel free to also ignore me as your son says he wants something quiet. I'm just highlighting other considerations for sniper secondaries as someone who has been doing it for 17 years or so.