do you really mean 4 and a half or 14 and a half years old?
if he’s only 4 and a half is he really safe on the road? or are you off roading?
its great he’s so keen either way, but don’t push him too hard
it’s oh so easy to put you son off
if you can, try borrowing a bike – he will grow so fast you will be hard pressed to keep buying new bikes each year or two