Fit is going to be a little more difficult than you may think. He's moving from a bike with shorter reach and steeper head tube angle with a slacker seat tube angle. Reach for a S is only 364mm.
Old last generation geometry.
Better new bikes have slacker head tube angles and longer reach with steeper seat tube angles.
15.5 Fuel has 415 or 420. Kona 27.5 S has 430 and RM has 430 S with 29 wheels or 433 for 27.5.
And he's had the major benefit of 40mm inner width rims and 27.5+ 2.8" tires. That cuts crashing and adds cornering traction. Makes climbing easier also. I don't see rims that wide on these fs bikes.
One option is a larger Roscoe 8. Your dealer should work a trade for you because of a growing kid.
There are better hardtails with current geo and a better fork. YouTube channel Hardtail Party has info/reviews and offers bike consulting. Link.
For fs I'd look for a 1st gen Evil Following. They ran small. Also an earlier gen Ibis Ripley.
Both have DW designed rear suspensions. Used they should be in your price range.
2017 Evil Following For Sale (pinkbike.com)
Also new an Intense 951 trail.
INTENSE 951 Series Trail