Guys
thanks for all the input, ideas and advice - am loving this community!
what I've decided is............
he sticks with piano lessons, plus I get him a bass of his own and I teach him some basics. If after 6 months - once the novelty factor has gone - and his heart is really with the bass, then he can drop piano and concentrate on bass.
so all I need to do now is find him a 1/2 or 3/4 scale bass - he is just too small to handle any of my full scale basses.
Thanks again & I will let you know how he gets on!
cheers
L