In that case, your son will report all the capital gain transactions associated with stock trades on his personal tax return.
As such, these transactions are reported on Schedule D, Capital Gains and Losses schedule. Assuming these transactions yield a net gain, then your son might have to pay capital gains tax on the profit.
However, it is very possible that your son's earned income in 2008 might qualify him to be in the zero percentage tax rate for Capital Gains tax in 2008
. In that case, he may not even be subject to any capital gains tax at all!