I pay my son in law for managing my IRA's and other investment portfolios. I simply write a monthly check to him. He is retired and has experience with investments. How should I handle the payment on my tax return? Can I deduct it as an expense? Is my check sufficient proof?

Probably OK. I think, however, that it's subject to the 2% of AGI miscellaneous deduction floor (i.e., no deduction until all miscellaneous deductions total at least the appropriate percentage). But, it would be income to your son, reportable on his tax return.