Why did he use "could" rather than "can"?
Native speaker’s answer
Rebecca
In this context, "can" and "could" both can be used. In general, "can" is present tense and "could" is past tense. However "could" can also be used when you are suggesting something to someone. Ex: Can you close the door? Ex: Could you close the door? Ex: You could go to sleep instead. Ex: You can go to sleep instead. "Could" is considered slightly more polite than "can", but there is no difference in meaning.