try giving him a target to fly to...for instance let him fly runway heading, then at the end of the runway have him make a hard heading change to aim at a distant tree or TV antenna..You can also have him do small loops the smaller the better...or teach him a snap roll....and above all else explain over and over again what he's doing wrong and that if he doesn't stop he can loose his plane as it flys off out of range....don't ever let him get away with his timid ways....
Let us know what happens