To what extent can I inform my US Manager about projects in Iran

We have no information on contractual obligations imposed on you by your employer. You may be compelled to provide information or prohibited from doing so on that basis. Conceivably there is some "compartmentalization" requirement to the effect that only certain employees are allowed access to certain information, so you'd need to ask higher-up in the company.

The company has the option of doing business in Iran, which may affect its ability to do business in the US. The company may not have the option of complying with US sanctions (I don't know the status of that move in the EU). The US Dept. of Treasury has a list of links pertaining to the Iran sanctions. From what I can determine, there is no law that imposes on US citizens a criminal or civil obligation to inform on foreign employers violating the sanctions, or to resign from such positions. On the other hand, it appears that a US person would have to wash their hands of such dealings. An example of one of the executive orders currently in force is this, which affects at least one German company. Section 1 says

All property and interests in property that are in the United States, that hereafter come within the United States, or that are or hereafter come within the possession or control of any United States person of the following persons are blocked and may not be transferred, paid, exported, withdrawn, or otherwise dealt in: any person determined by the Secretary of the Treasury, in consultation with the Secretary of State

(descriptions which basically identify anyone in mining). On the face of it, the manager is compelled to not engage in such transactions.

If the EU blocking statute is in force, the manager probably cannot comply with US law, and yet he must under US law. So I would say, this is a legally unanswerable question. I doubt that there is any provision of German law that would penalize you for telling the manager of his legal risks, but I would hire an attorney to protect myself if I were in that position.