Not claiming to know much here, but first are your sure it is a joint account and not a custodial account? The latter would be a LOT more rational and I think easier.
The other point is that if you do not sell the stock prior to transferring, then it will be simpler as there are no taxes prior to transfer. You will have to record the gift tax, but you do not pay anything as it is only a debit against your $11M lifetime limit.
No, I believe the transfer would be considered a gift of your 50% ownership in the account. If that 50% is worth less than 15,000 then it is exempt from any gift tax consequences. For federal income tax purposes, the gift would NOT be an expense to you or income to your son. For capital gains tax purposes your son would keep your basis in the stock and pay the applicable capital gains in the year he sells. (Either short term or long term.) Hope that helps/makes sense.
News 
Market Data 
Discover 
