Looking at the tax laws, a notary is not considered a "Trade or Business" (and therefore not subject to SE tax) because it is a "Public Office".  When it is directly in connection with his attorney business, that MIGHT void the "Public Office" exemption from SE tax.
https://www.law.cornell.edu/uscode/text/26/1402#c
https://www.law.cornell.edu/cfr/text/26/1.1402%28c%29-2