i work for a broker and we do not allow anyone under 18 to hold shares - however, other brokers may allow this practise, but not as far as i know.
the way it normally works in a nominee account is that the name of the account is:
Parents Name A/c Childs Name
The child cannot give any instructions or have any dealings on the account. however, once the child turns 18, the parent is automatically removed from the account.
You just open a normal sharedealing account and then you need to provide a copy of your son's passport/birth cert.
Not a tax expert so not sure on that aspect.