I just built my 8 year old his first 26″ wheeled bike (he is tall). Cost £350 in all and is build around a tiny rockhopper frame. It has shimano slx disc brakes, rock shock duke air forks and dt swiss wheels. Finished off with deore drivetrain and some new schwalbe tyres from on one. All stuff bought used, mostly from here.
For a 14 year old you will have more choice in frames and forks as he will be taller and heavier than my son (we needed short travel light air ones for my son).
Build one with him i’d say, my son loved collecting all the bits (which I of course colour coordinated, being a girl), and he helped building it. I did most of it and took it to the bike shop to finish tuning the gears.
His bike rides lovely and is much better than anything else you can get for that money.
Simone