The Cometa 50 is an excellent little pint sized springer for about £70, several other spring gun manufacturers also do similiar guns for youngsters. (Have only ever experienced the Cometa though!)

Learning to shoot with an open sighted springer will be good for him, and cheaper for you (no gas etc.). As he gets older and wants to shoot more it will be a simple matter to upgrade to Meteors, supersports etc. etc.

Alex