Im 14 and have almost stopped biting my nails.
To make them taste bad you dip them in soapy water and let them air dry (not sure if it works as I havent tried it before)
Give him something to play with like silly putty, a little ball or even a coin.
What I found worked best for me is wearing a rubber band around my wrist and every time I bit my nails I snapped the rubber band enough to make it really hurt and eventually when I learned that if I bite my nails it would be sore i stopped.
The best thing to do though is BE DETERMINED!