ive got a stomp klx with a yx140 engine, upgraded carb, race cammed...like shit off a stick, and a whole lot of fun. real reliable too. plus when things do go wrong pit bikes are cheap to fix. not being too sure how old the bairn is, id say, get him either a semi auto, or a rev and go to start. then once hes got to grips, buy a decent bike. a geared bike. if hes around the 12 mark, then you can probably get away with getting a gears one straight away. stomp are good, demon, an thumpstar if you can find them but they're becoming increasingly rare.
Just be careful putting him in at the deep