I am not a Pennsylvania attorney and so cannot give any actual legal advice in this matter. But I have spent my career in the criminal justice system and so have some thoughts about how you might want to consider proceeding.
As others have suggested, it is often a good idea to contact local law enforcement. District Attorney's offices typically cannot take action without first receiving a referral from the police. I would start by figuring out in which jurisdiction the crime actually occurred. In this case, your best shot is probably the city where he was working at the time that the knife went missing, since it is reasonable to believe that he used the knife at work ("probable cause" is typically defined as a "reasonable belief," under the totality of the circumstances, that a crime has been committed). If you can figure out which precinct the restaurant is in, you can call the precinct and explain that you need to file a theft report, and want to know the appropriate way to do that. You may also let them know that you have information that the suspect has since changed work sites. If the new restaurant is in a different jurisdiction, they may advise you to file the report there instead. Because you have good suspect information, one of those municipal police agencies will likely dispatch a patrol officer to contact the suspect at his place of employment.
Beyond that, here is a little background on what appears to be the relevant criminal provisions. In Pennsylvania, a person is guilty of "theft" of moveable property if the person "unlawfully takes, or exercises unlawful control over, movable property of another with intent to deprive him thereof." 18 Pa.C.S. § 3921(a). To "deprive" means either (1) "[t]o withhold property of another permanently or for so extended a period as to appropriate a major portion of its economic value;" or (2) "to dispose of the property so as to make it unlikely that the owner will recover it." 18 Pa.C.S. § 3901.
There is also the crime in Pennsylvania of theft by "deception." That crime occurs when a person "intentionally obtains or withholds property of another by deception." 18 Pa.S.C. § 3922(a). A person "deceives" another within the meaning of that section if the person "intentionally * * * creates or reinforces a false impression, including false impressions as to law, value, intention or other state of mind; but deception as to a person's intention to perform a promise shall not be inferred from the fact alone that he did not subsequently perform the promise." 18 Pa.S.C. § 3922(a)(1).
As relevant here, theft is a third-degree felony if the value of the stolen property exceeds $2,000. 18 Pa.C.S. § 3903(a.1). If the value is less than $2,000, but more than $200, it is a first-degree misdemeanor. 18 Pa.C.S. § 3903(b). If the value is between $50 and $200, it is a second-degree misdemeanor. 18 Pa.C.S. § 3903(b)(1). Less than $50 is a third-degree misdemeanor. 18 Pa.C.S. § 3903(b)(2).
The "value" of the stolen property "means the market value of the property at the time and place of the crime, or if such cannot be satisfactorily ascertained, the cost of replacement of the property within a reasonable time after the crime." 18 Pa.C.S. § 3903(c)(1).
Again, I cannot give legal advice in this matter but hopefully that provides some potentially useful educational starting points for looking into the issue further.