The American Bar Association has promulgated ethical standards for over a century:
https://www.americanbar.org/publications/youraba/2016/december-2016/a-brief-history-of-the-development-of-legal-ethics-standards-in-.html
The ABA, a voluntary trade organization, cannot enforce the standards but enforcement can come from state or federal judiciaries or bar associations that adopt or amend the ABA standards (or draft their own ethical standards).
I suspect Scott Boras offers a prospective client a detailed contract that outlines the respective responsibilities of the agent and the player. I am almost certain that the contract specifically states that the player hires Scott Boras as an agent, not as an attorney. I would not be surprised if the contract has a clause that advises the player to consult an attorney if he need legal advice.
Boras maintains an active membership in the State Bar of California:
http://members.calbar.ca.gov/fal/Member/Detail/108792
... but many ethical standards do not extend to his work as an agent (not as an attorney, although the line can be blurred). I am reminded of my real estate agent, an active member of the state bar, who as a realtor could represent multiple clients with inconsistent interests.
The Major League Baseball Players Association regulates player agents:
http://www.mlbplayers.com/pdf9/4925108.pdf
... including a certification system that authorizes disciplinary action for violations.
In the highly competitive field of MLB agency, reports of ethical violations might be more likely to come from a fellow agent than from an MLB front office. Because of his celebrity status, I suspect Scott Boras is the most scrutinized agent in baseball.