I get the impression that you think your gift to your son has to be reported on your 1040 tax return. It is not.
Gifts are not reported on any tax return, ever. Additionally, the name of the form is misleading. *NOBODY* pays tax on a gift. Period. This IRS just has reporting requirements for a gift given in any one tax year, if the value of that gift exceeds $15K. All the giver is required to do is report it to the IRS on the form 709. That's it.
In your case, the value of the gift given to your son in a single tax year is $40,000. So your spouse gifted him $20K and you gifted him 20K. Each of you needs to file your own separate IRS Form 709.
The form 709 is *NOT* filed with your 1040 tax return. It gets mailed separately to the IRS to the address in the instructions. If you need help with the actual form itself, I don't think you'll find many in this forum that can help. But the instructions for the 709 can be found at https://www.irs.gov/pub/irs-pdf/i709.pdf The address you mail it to is also in those instructions. (It may not be the same place you would mail your return to.)
