Gambling winnings are fully taxable and must be reported on your tax return. There is no minimum amount to report this type of income.
Assuming your son is not your dependent, filing as single with standard deduction - his filing requirements are based on the total income $8450 (for 2006).
As his income is above this amount - he is required to file and the actual taxes will be based on his tax bracket.
let me know if any clarification needed.