I am going have someone who know more about payroll tax to answer your question.
there is no minimum pay to avoid income tax liability - any payment should be included into person's income for income tax purposes - that income should be added to other taxable income and tax liability is calculated based on the total income.
If the total income is below certain level - there is no federal tax liability.
For single person who is not a dependent - such level is $5700+$3650=$9350 (for 2009)
If these people are paid as employees - your son should issue W2 form to them at the end of the year and withhold FICA taxes regardless of the amount they are paid.
There is no minimum pay limit.
If these people are self-employed contractors - you son do not withhold any taxes from the paycheck and issue the form 1099-misc at the end of the year.
He doesn't require to issue that form if the person is paid less than $600 per entire year.
However - the recipient still need to report that amount on his/her tax return and it is a subject of income tax.
Let me know if you need any help.