I would definitely consult a lawyer beforehand. The letter can indeed have consequences. Namely, that the contractor has timely pointed out the delay and is thereby released from the obligations in this regard. Especially since he speaks of force majeure, the fault then does not lie with him.
Yes, especially if he can prove receipt (also implicitly, e.g., if he is demonstrably contacted afterwards), one should probably not let his claim of force majeure go unchallenged.
At a minimum, I would require him to explain the reasons for the delay since the contract signing. So, which circumstances have changed since the signing.
Thank you for your willingness to help, it's nice to see that you are helping so selflessly! I have a consultation appointment with the lawyer tomorrow. I don't want to make a big deal out of it, just not to make any mistakes. I will report back!