Both are correct, the first one implies that he will not be able to come (talking from the present in reference to the future), the second one implies that he was not able to come (talking from the future in reference to the past).

Both sentences can be correct 🙂 “He called to say he can’t come.” → This is used when the situation is still true now (he still can’t come). “He called to say he couldn’t come.” → This is more common in reported speech, especially when you are telling the story later. In everyday English, “He called to say he couldn’t come” sounds more natural when you are reporting what he said. So grammatically both work, but the second one is usually preferred. 👍