I assume for purposes of this answer that boyfriend was an at-will employee who was not a union member working under a collective bargaining agreement, and not covered by an employee manual providing that employees will not be discharged without good cause. As a general principle of law, at-will employees have very few protections against discharge. An at-will employee generally may be fired for any reason or no reason at all, unlawful discrimination and unlawful retaliation excepted.
The comment you describe in this case was both sexualized and offensive. A single offensive comment is usually insufficient to give rise to a hostile-work-environment sexual harassment claim under federal antidiscrimination law, or under state laws which are usually patterned on, and look to, federal antidiscrimination law.
Antidiscrimination law as it applies to a "hostile work environment" evolved to address circumstances in which a workplace was pervaded by sexual comments, conduct or stimuli such that working conditions were markedly different depending upon whether the employee is male or female. However, the reach of antidiscrimination law barring sexual harassment in the workplace was extended by the United States Supreme Court to cover male-on-male sexual harassment. The leading case, Oncale v. Sundowner Offshore Services, decided in 1998 (see link below) involved much more than a single offensive comment; the plaintiff in Oncale was sodomized with a bar of soap and threatened with rape. The tortfeasors and the victim in that case were all heterosexual; the conduct at issue was purely harassive.
The circumstance you describe I interpret as an instance of male-on-male sexual harassment a la Oncale v. Sundowner Offshore Services, but you only report a single offensive comment by a supervisor, followed by a discharge. Usually only a pervasively sexually harassive workplace will be held to be discrimination "because of ... sex" under Title VII, and under state law patterned after Title VII.
Every state's law is different, and if your boyfriend thinks he may want to pursue a claim he may want to consult counsel licensed in Ohio who handles employment discrimination cases on behalf of Plaintiffs. Because the conduct involved a single offensive comment, however, I cannot be encouraging on the prospect of a successful claim.
Not legal advice as I don't practice law in Ohio. It's just my two cents on the facts you describe in light of general principles of antidiscrimination law. If you need legal advice, please consult a lawyer who holds Ohio licensure. That's not me as I practice in Vermont ONLY.
I agree with my Vermont colleague in that one incident such as this does not rise to the level of sexual harassment. I disagree in one respect. The offensive comment, although using a genital referenc,e is not necessarily based on "sex" which is required by statute. Which means, on its face, this does not look like a winnable case.
This post is not meant to: 1) contain my signature; 2) contain legal advice; 3) create an attorney/client relationship; or 4) guarantee confidentiality.