Just amend your Bill of Sale to include that he is signing to attest to the fact that he is not a prohibited person. An example of the statement is in 18 USC 922 (c)(1):
“Subject to penalties provided by law, I swear that, in the case of any firearm other than a shotgun or a rifle, I am twenty-one years or more of age, or that, in the case of a shotgun or a rifle, I am eighteen years or more of age; that I am not prohibited by the provisions of chapter 44 of title 18, United States Code, from receiving a firearm in interstate or foreign commerce; and that my receipt of this firearm will not be in violation of any statute of the State and published ordinance applicable to the locality in which I reside.
Signature XXXXXXXXX Date XXXX.”