play w/ the maps! the 21 they moved a lot of the power from down low to try to get it more mid-top and it lost a lot on bottom. Coming from a 450 I missed that. Your son coming from a 125 he won't notice any difference, but definitely check the yamaha site and setup your power tuner app with the maps they have (and you can find some more if you look at some of the testing/review sites like keefer for example) theres a free rev map and a top end map, and a low end map.. My fav so far I think is the low end map, I'm leaving that & the top end map programmed in to give me 2 options depending on the track..even though it takes about 30 seconds to swap them out from your phone. Coming from a 2 stroke he probably won't like the engine breaking so much, the free revving map gets rid of some of that compared to the stock map for sure.
ive only got 1.9 hours on it, so I'm still adjusting.. but very happy with my 21 yz250f right outta the box. made a few minor clicker adjustments but even on stock settings it felt great. yamaha has done a hell of a job with that bike the past few years and I think it's got even better in 21.