if you can go up to $1000, options open up, especially if you can find a 2019, like this dude:
https://www.pedalonline.com/product/...5-346084-1.htm
For under 800 bucks, Specialized does a couple 1x's, like the Pitch Comp:
https://www.bikesourceonline.com/pro...x-364146-1.htm
and the Rockhopper Comp 1x:
https://www.bikesourceonline.com/product/specialized-rockhopper-comp-1x-364152-1.html
Ironically, the 1x drivetrains are more expensive than the 2x8s and 2x9s. Something to keep in mind, too, is that the 2x9 will actually have a wider gearing. If you go with a good shop, minor adjustments should be free (or at least the first one after the cables stretch and settle in), so I wouldn't worry too much about 1x vs 2x. If it were my son, I'd be looking at the Pitch Comp (but that's my son)...
good luck, happy bday to him!