In the past someone may have fitted him up with too fierce a bit and if the rider was also heavy handed then that would have encouraged the horse to move backwards to evade the pain. The backing up then became a habit.
Let someone check your seat especially whether you sit upright at 90 degrees and that your hands are light, especially at the halt. Make sure the bit in use is not too fierce for your purposes.
Check the saddle that it is sitting horizontal. Make sure it is not digging at the back or the front end
The longer he has been doing it, the more difficult to erase. Your chap is 16 - he won't forget now.
You,ve got to send him forwards immediately he hints of going backwards. A experienced rider used to wearing spurs might help you to cure the chap - ie the prick of the spur would send him forwards but if you are inexperienced with spurs you might do more harm than good.
As the lady said - maybe worth a few dollars for a professional's help. You don't need that type of evasion in the wrong environment.