Giving rental property to my son who already owns rental properties, however don't want to tie up his credit with the small equity line on that property by keeping it in my name for now ----want to give him full benefit of the tax, insurance, expenses of the property... Is it possible to give him a 1099 or do I need to write a mortgage or rent to own

You can not keep the property in your name and allow him the benefit of taxes. Since you own it. If there is a current loan, you can not deed it to him without paying that off first.
Even a rent to own or contract for deed, does not give him full rights to claim property taxes, that is still you as the owner.

He can as a rent to own or contract for deed, own it as a rental property and write off all normal business expenses.